fix(console): add CORS headers and OPTIONS preflight to Zen/Go API endpoints

[G4brym]

Issue for this PR

Closes #31041

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The Zen/Go API endpoints return 404 on CORS preflight (OPTIONS) requests because only /zen/v1/models and /zen/go/v1/models had OPTIONS handlers. Browser clients were blocked from calling the chat completions, messages, and responses endpoints.

This PR:

• Adds a shared corsHeaders constant in packages/console/app/src/routes/zen/util/cors.ts.
• Adds OPTIONS handlers to all Zen/Go inference routes:
  • /zen/v1/chat/completions
  • /zen/v1/messages
  • /zen/v1/responses
  • /zen/v1/models/[model]
  • /zen/go/v1/chat/completions
  • /zen/go/v1/messages
  • /zen/go/v1/responses
• Adds CORS headers to all POST/streaming/error responses from packages/console/app/src/routes/zen/util/handler.ts.
• Adds CORS headers to the GET /zen/v1/models and GET /zen/go/v1/models responses.

How did you verify your code works?

• bun test cors.test.ts in packages/console/app passes (5 tests).
• bun test in packages/console/app passes (12 tests across all console app test files).
• bun run typecheck in packages/console/app passes.

Screenshots / recordings

N/A - no UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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