fix: add CORS headers to early-return responses in chat completions route

[Copilot]

The 400 (prompt injection blocked) and 503 (guard unavailable) early-return responses in the chat completions route were missing CORS headers, causing browser clients to be blocked by CORS on those paths even though the server returned valid JSON.

Changes

• src/app/api/v1/chat/completions/route.ts: Import CORS_HEADERS from the shared cors utility and spread into both early-return response headers

// Before
{ status: 503, headers: { "Content-Type": "application/json" } }

// After
{ status: 503, headers: { ...CORS_HEADERS, "Content-Type": "application/json" } }

Applies to both the 400 injection-blocked response and the 503 guard-unavailable response, consistent with every other API route in the codebase.

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[diegosouzapw]

Fix integrated directly into features-agente-mcp-a2a branch. Thank you Copilot! 🎉