* Fix #375: support OpenAPI 3.1 array-valued type (unions & nullable)

OpenAPI 3.1 / JSON Schema 2020-12 allows a schema's `type` to be an array,
e.g. type: ["string", "null"] (the nullable idiom that replaces 3.0's
`nullable: true`, and general unions). ApiTool.getType() read the type via
JsonNode.textValue(), which returns null for an array node and collapsed to
"", so every isObject/isArray/isString/isNumber/isDateTime predicate and
MapperUtil.getSimpleType failed -> the generator silently produced wrong or
empty types for those fields.

- ApiTool.getType: when `type` is an array, resolve to the first non-"null"
  entry ("null" only marks the type as nullable). All the isX predicates and
  getSimpleType then work unchanged for union/nullable types.
- MapperUtil.processNumber: read the type via ApiTool.getType instead of
  schema.get("type").asText(), so array-valued numeric types resolve.
- Add TypeConstants.NULL.
- Add a 3.1.0 regression fixture (testOpenApi31Types) covering ["string",
  "null"], ["integer","null"], ["number","null"]+double, int64 and a plain
  array, with golden assets.

Bumps version 6.3.2 -> 6.4.0 across the engine, maven and gradle modules.
Non-array `type` handling is unchanged, so existing 3.0 specs are unaffected.

Follow-up 3.1 items (examples array, $ref siblings, webhooks) tracked in #375.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Support OpenAPI 3.1 top-level webhooks (#375)

OpenAPI 3.1 adds a top-level `webhooks` object: a map of named Path Item
Objects describing out-of-band requests. The generator only iterated
`paths`, so webhooks were ignored - no handler interface, no payload models.

Add OpenApiUtil.mergeWebhooksIntoPaths, invoked right after parsing in
OpenApiGenerator.processFile. Each webhook (keyed by name) is merged into
`paths` under a "/"-prefixed key so the existing pipeline generates a
handler interface for its operations and the request/response payload
models. `paths` is created if the contract has none, and existing `paths`
entries are never overwritten.

Adds a 3.1.0 webhooks-only regression fixture (testWebhooks) with golden
assets (NewPetApi + PetDTO).

Stacked on the 6.4.0 array-type work (#376); both are part of the 3.1
support milestone, so the version stays 6.4.0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Default tags on tag-less webhook operations to avoid NPE

Webhook operations normally omit `tags`, but createOperation requires
one (Objects.requireNonNull), so a realistic tag-less webhook crashed
with a NullPointerException in the default by-url grouping mode - and
was silently skipped in tag-grouping mode. The prior test masked this
by giving the webhook explicit tags.

mergeWebhooksIntoPaths now defaults each webhook operation's tags to the
webhook name when absent/empty, and skips blank keys (guards the
pathUrl.split()[1] grouping). The testWebhooks fixture is updated to the
realistic tag-less shape to cover this.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Harden 3.1 type/webhook handling and add edge-case tests

getType: extract array-type resolution into a helper. A genuine multi-type
union (e.g. ["string","integer"]) now logs a warning and uses the first
concrete type; a "null"-only/empty type array falls back to `object`
instead of an empty type string.

Adds edge-case regression fixtures:
- testOpenApi31Union: union -> String (+warning), ["null"] -> Object
- testWebhookPathCollision: webhook name colliding with an existing path
  is dropped (existing paths take precedence).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Annotate Gradle tasks with @DisableCachingByDefault

Gradle 9.x plugin validation fails the build when a task type is neither
@CacheableTask nor @DisableCachingByDefault. OpenApiTask and AsyncApiTask
are code generators whose spec-file inputs are not declared as cacheable
inputs, so caching is disabled explicitly with a documented reason.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* fix: update thollander/actions-comment-pull-request to v2 (Node 20 deprecated)

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Co-authored-by: joseegarcia <jose.garcia@disashop.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>