Include LLM headers in ModelConfig (browserbase#1874)

# why
Custom headers for LLM requests are supported in Stagehand, canonical
SDKs were not accounting for this until now.

# what changed
Added headers to ModelConfig

# test plan



<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Adds an optional `headers` field to `ModelConfig` to send custom headers
with every LLM request. Also includes a patch changeset for
`@browserbasehq/stagehand`.

- **New Features**
- Add `headers?: Record<string, string>` to `ModelConfigObjectSchema` in
`packages/core/lib/v3/types/public/api.ts`.
- Update `packages/server-v3/openapi.v3.yaml` to document `headers` in
`ModelConfig`.
- Update `packages/server-v4/openapi.v4.yaml` to use selector-based
result/output schemas for page actions (click, hover, scroll,
drag-and-drop) and add `returnSelector` support.

<sup>Written for commit f83bd8b.
Summary will update on new commits. <a
href="https://cubic.dev/pr/browserbase/stagehand/pull/1874">Review in
cubic</a></sup>

<!-- End of auto-generated description by cubic. -->

---------

Co-authored-by: Sean McGuire <seanmcguire1@outlook.com>

Author: miguelg719

.changeset/odd-grapes-glow.md

@@ -0,0 +1,5 @@
+---
+"@browserbasehq/stagehand": patch
+---
+
+Add headers (LLM) to ModelConfig

packages/core/lib/v3/types/public/api.ts

@@ -79,6 +79,10 @@ export const ModelConfigObjectSchema = z
       description: "Base URL for the model provider",
       example: "https://api.openai.com/v1",
     }),
+    headers: z.record(z.string(), z.string()).optional().meta({
+      description:
+        "Custom headers sent with every request to the model provider",
+    }),
   })
   .meta({ id: "ModelConfigObject" });
 

packages/server-v3/openapi.v3.yaml

@@ -193,6 +193,13 @@ components:
           example: https://api.openai.com/v1
           type: string
           format: uri
+        headers:
+          description: Custom headers sent with every request to the model provider
+          type: object
+          propertyNames:
+            type: string
+          additionalProperties:
+            type: string
       required:
         - modelName
     ModelConfig:
@@ -1225,6 +1232,13 @@ components:
           example: https://api.openai.com/v1
           type: string
           format: uri
+        headers:
+          description: Custom headers sent with every request to the model provider
+          type: object
+          propertyNames:
+            type: string
+          additionalProperties:
+            type: string
       required:
         - modelName
       additionalProperties: false