feat: add Spector integration tests for Payload_Head and Payload_JsonMergePatch

[Copilot]

Adds Spector integration test coverage for two previously untested scenario groups: Payload_Head and Payload_JsonMergePatch.

Changes

• test/commands/spector-list.js: Added payload/head and payload/json-merge-patch entries to the azureModularTsps array.
• test/azureModularIntegration/generated/payload/head/: Added tspconfig.yaml, .gitignore, and generated src/index.d.ts for the HeadClient.
• test/azureModularIntegration/generated/payload/json-merge-patch/: Added tspconfig.yaml, .gitignore, and generated src/index.d.ts for the JsonMergePatchClient.
• test/azureModularIntegration/payloadHead.spec.ts: New spec covering contentTypeHeaderInResponse (HEAD request; mock server validates 200).
• test/azureModularIntegration/payloadJsonMergePatch.spec.ts: New spec covering createResource (PUT with full body), updateResource, and updateOptionalResource (PATCH with JSON merge-patch null field-removal semantics).

Known emitter limitations

• HEAD headers not assertable: HeadClient.contentTypeHeaderInResponse() returns Promise<void>; response headers (Content-Type, x-ms-meta) are not surfaced by the emitter. Server validation on 200 is the effective assertion.
• Nullable merge-patch fields: The generated ResourcePatch type uses ? (optional) fields rather than T | null, so as any casts are required to pass null values for field-removal. The serializer preserves null at runtime correctly.

[v-jiaodi]

@copilot create issue for it.

[Copilot]

I don't have the ability to create GitHub issues directly. Please create an issue with the following details:

Title: [Emitter] HEAD operations should surface response headers in the return type

Description:
The TypeSpec emitter generates Promise<void> for HEAD operations (e.g. HeadClient.contentTypeHeaderInResponse()), which means response headers (Content-Type, x-ms-meta) are not accessible from the returned value. This is an emitter limitation — the TypeSpec spec defines these headers as part of the response, but the generated SDK discards them.

Scenario: Payload_Head / contentTypeHeaderInResponse (payload/head/content-type-header-in-response)

Expected behavior: The generated method should return an object containing the response headers, e.g.:

{ contentType?: string; metadata?: string }

Actual behavior: Returns Promise<void>, making header assertions impossible in integration tests.

Related test: packages/typespec-ts/test/azureModularIntegration/payloadHead.spec.ts