Backport HttpBody/HttpServerResponse contentType improvements

[tim-smart]

Summary

• Backport Effect 4 content-type behavior for HttpBody by adding optional contentType support to json, unsafeJson, jsonSchema, and urlParams.
• Update HttpServerResponse JSON and URL-encoded constructors to consistently derive content type from options.contentType, then options.headers[\"content-type\"], then defaults.
• Add regression tests for custom content-type handling and include a patch changeset for @effect/platform.

Validation

• pnpm lint-fix
• pnpm test run packages/platform/test/HttpBody.test.ts packages/platform/test/HttpApp.test.ts
• pnpm check
• pnpm build
• pnpm docgen

Closes #6046

[changeset-bot]

🦋 Changeset detected

Latest commit: 2625461

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 31 packages

Name	Type
@effect/platform	Patch
@effect/cli	Patch
@effect/cluster	Patch
@effect/experimental	Patch
@effect/opentelemetry	Patch
@effect/platform-browser	Patch
@effect/platform-bun	Patch
@effect/platform-node-shared	Patch
@effect/platform-node	Patch
@effect/rpc	Patch
@effect/sql-clickhouse	Patch
@effect/sql-d1	Patch
@effect/sql-drizzle	Patch
@effect/sql-libsql	Patch
@effect/sql-mssql	Patch
@effect/sql-mysql2	Patch
@effect/sql-pg	Patch
@effect/sql-sqlite-bun	Patch
@effect/sql-sqlite-node	Patch
@effect/sql	Patch
@effect/workflow	Patch
@effect/ai	Patch
@effect/ai-amazon-bedrock	Patch
@effect/ai-anthropic	Patch
@effect/ai-google	Patch
@effect/ai-openai	Patch
@effect/ai-openrouter	Patch
@effect/sql-sqlite-do	Patch
@effect/sql-sqlite-react-native	Patch
@effect/sql-sqlite-wasm	Patch
@effect/sql-kysely	Patch

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