fix(theme): handle nested discriminators in allOf structures

Add support for discriminators nested within allOf schemas. Previously,
discriminators inside allOf items were not properly detected, and the
discriminator property lookup only checked top-level schema properties.

Changes:
- Add findProperty() helper to recursively search for properties in
  nested oneOf/anyOf/allOf structures
- Update DiscriminatorNode to use findProperty() for discriminator
  property lookup
- Update SchemaNode to check for discriminators in allOf items and
  merge schemas before processing
- Return null instead of empty "object" SchemaItem when properties
  are empty after discriminator processing
- Add test case for nested discriminator in allOf

This fixes scenarios like:
```yaml
allOf:
  - $ref: CommonProps
  - oneOf: [VariantA, VariantB]
    discriminator:
      propertyName: type
```

Fixes #1302

Author: sserrata

demo/examples/tests/discriminator.yaml

@@ -350,6 +350,73 @@ paths:
               schema:
                 $ref: "#/components/schemas/BaseEmptySubschema"
 
+  /discriminator-nested-allof:
+    post:
+      tags:
+        - discriminator
+      summary: Nested Discriminator in allOf
+      description: |
+        Tests discriminators nested within allOf structures, where the discriminator
+        and its property definition are inside an allOf item rather than at the top level.
+        This is a common pattern when combining shared properties with discriminated unions.
+
+        Schema:
+        ```yaml
+        allOf:
+          - $ref: '#/components/schemas/CommonProps'
+          - oneOf:
+              - $ref: '#/components/schemas/NestedVariantA'
+              - $ref: '#/components/schemas/NestedVariantB'
+            discriminator:
+              propertyName: variantType
+              mapping:
+                variant-a: '#/components/schemas/NestedVariantA'
+                variant-b: '#/components/schemas/NestedVariantB'
+        ```
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: "#/components/schemas/NestedDiscriminatorBase"
+      responses:
+        "200":
+          description: Successful response
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/NestedDiscriminatorBase"
+
+  /discriminator-doubly-nested:
+    post:
+      tags:
+        - discriminator
+      summary: Doubly Nested Discriminators (Issue #1302 Full Scenario)
+      description: |
+        Tests the full scenario from issue #1302 with doubly-nested discriminators:
+        - Outer level: allOf with discriminator on "type" selecting TypeA/TypeB/TypeC
+        - TypeA: has allOf with its own nested discriminator on "mode"
+        - TypeB: has allOf with its own nested discriminator on "mode"
+        - TypeC: has discriminator on "mode" at root level (NOT in allOf)
+
+        This tests:
+        1. Discriminators nested in allOf are properly detected
+        2. Inner discriminators work after selecting outer type
+        3. TypeC pattern (discriminator at root) still works
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: "#/components/schemas/DoublyNestedBase"
+      responses:
+        "200":
+          description: Successful response
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/DoublyNestedBase"
+
 components:
   schemas:
     BaseBasic:
@@ -569,3 +636,227 @@ components:
       type: object
       allOf:
         - $ref: "#/components/schemas/BaseEmptySubschema"
+
+    # Schemas for nested discriminator test
+    CommonProps:
+      type: object
+      properties:
+        id:
+          type: string
+          format: uuid
+          description: Unique identifier
+        createdAt:
+          type: string
+          format: date-time
+          description: Creation timestamp
+      required:
+        - id
+
+    NestedDiscriminatorBase:
+      allOf:
+        - $ref: "#/components/schemas/CommonProps"
+        - oneOf:
+            - $ref: "#/components/schemas/NestedVariantA"
+            - $ref: "#/components/schemas/NestedVariantB"
+          discriminator:
+            propertyName: variantType
+            mapping:
+              variant-a: "#/components/schemas/NestedVariantA"
+              variant-b: "#/components/schemas/NestedVariantB"
+
+    NestedVariantA:
+      type: object
+      properties:
+        variantType:
+          type: string
+          enum: ["variant-a"]
+        configA:
+          type: object
+          properties:
+            settingA1:
+              type: string
+            settingA2:
+              type: integer
+      required:
+        - variantType
+
+    NestedVariantB:
+      type: object
+      properties:
+        variantType:
+          type: string
+          enum: ["variant-b"]
+        configB:
+          type: object
+          properties:
+            settingB1:
+              type: boolean
+            settingB2:
+              type: array
+              items:
+                type: string
+      required:
+        - variantType
+
+    # Test case for doubly-nested discriminators (issue #1302 full scenario)
+    # Outer: allOf with discriminator on "type"
+    # Inner: TypeA and TypeB each have their own discriminator on "mode"
+    DoublyNestedBase:
+      allOf:
+        - $ref: "#/components/schemas/OuterCommonProps"
+        - oneOf:
+            - $ref: "#/components/schemas/OuterTypeA"
+            - $ref: "#/components/schemas/OuterTypeB"
+            - $ref: "#/components/schemas/OuterTypeC"
+          discriminator:
+            propertyName: type
+            mapping:
+              type-a: "#/components/schemas/OuterTypeA"
+              type-b: "#/components/schemas/OuterTypeB"
+              type-c: "#/components/schemas/OuterTypeC"
+
+    OuterCommonProps:
+      type: object
+      properties:
+        id:
+          type: string
+          format: uuid
+        name:
+          type: string
+
+    # TypeA: has allOf with its own nested discriminator
+    OuterTypeA:
+      title: TypeA
+      allOf:
+        - $ref: "#/components/schemas/InnerCommonProps"
+        - oneOf:
+            - $ref: "#/components/schemas/ModeA1"
+            - $ref: "#/components/schemas/ModeA2"
+          discriminator:
+            propertyName: mode
+            mapping:
+              mode-a1: "#/components/schemas/ModeA1"
+              mode-a2: "#/components/schemas/ModeA2"
+      properties:
+        type:
+          type: string
+          enum: ["type-a"]
+      required:
+        - type
+
+    # TypeB: also has allOf with its own nested discriminator
+    OuterTypeB:
+      title: TypeB
+      allOf:
+        - $ref: "#/components/schemas/InnerCommonProps"
+        - oneOf:
+            - $ref: "#/components/schemas/ModeB1"
+            - $ref: "#/components/schemas/ModeB2"
+          discriminator:
+            propertyName: mode
+            mapping:
+              mode-b1: "#/components/schemas/ModeB1"
+              mode-b2: "#/components/schemas/ModeB2"
+      properties:
+        type:
+          type: string
+          enum: ["type-b"]
+      required:
+        - type
+
+    # TypeC: has discriminator at root (NOT in allOf)
+    OuterTypeC:
+      title: TypeC
+      type: object
+      oneOf:
+        - $ref: "#/components/schemas/ModeC1"
+        - $ref: "#/components/schemas/ModeC2"
+      discriminator:
+        propertyName: mode
+        mapping:
+          mode-c1: "#/components/schemas/ModeC1"
+          mode-c2: "#/components/schemas/ModeC2"
+      properties:
+        type:
+          type: string
+          enum: ["type-c"]
+      required:
+        - type
+
+    InnerCommonProps:
+      type: object
+      properties:
+        timestamp:
+          type: string
+          format: date-time
+
+    ModeA1:
+      type: object
+      properties:
+        mode:
+          type: string
+          enum: ["mode-a1"]
+        settingA1:
+          type: string
+      required:
+        - mode
+
+    ModeA2:
+      type: object
+      properties:
+        mode:
+          type: string
+          enum: ["mode-a2"]
+        settingA2:
+          type: number
+      required:
+        - mode
+
+    ModeB1:
+      type: object
+      properties:
+        mode:
+          type: string
+          enum: ["mode-b1"]
+        settingB1:
+          type: boolean
+      required:
+        - mode
+
+    ModeB2:
+      type: object
+      properties:
+        mode:
+          type: string
+          enum: ["mode-b2"]
+        settingB2:
+          type: array
+          items:
+            type: string
+      required:
+        - mode
+
+    ModeC1:
+      type: object
+      properties:
+        mode:
+          type: string
+          enum: ["mode-c1"]
+        settingC1:
+          type: integer
+      required:
+        - mode
+
+    ModeC2:
+      type: object
+      properties:
+        mode:
+          type: string
+          enum: ["mode-c2"]
+        settingC2:
+          type: object
+          properties:
+            nested:
+              type: string
+      required:
+        - mode