fix: add validateBody + structured 400 error handling to /api/combos/auto

[Copilot]

POST /api/combos/auto called req.json() without validateBody, failing the check-route-validation.mjs CI guard, and returned malformed-JSON errors as 500 with String(err) exposed to the client.

Changes

• src/shared/validation/schemas.ts — Added createAutoComboSchema (with a scoringWeightsSchema helper) covering all auto-combo fields with appropriate range/length constraints
• src/app/api/combos/auto/route.ts — Refactored POST handler:
  • JSON parsing isolated in its own try/catch; malformed input returns a structured 400 matching the { error: { message, details } } shape used by all other validated routes
  • Body validated via validateBody(createAutoComboSchema, rawBody) + isValidationFailure
  • Generic 500 catch no longer leaks String(err) to the client

// Before
const body = await req.json();           // no validation
return NextResponse.json({ error: String(err) }, { status: 500 }); // leaks on bad JSON

// After
rawBody = await req.json();              // isolated try/catch → structured 400
const validation = validateBody(createAutoComboSchema, rawBody);
if (isValidationFailure(validation)) {
  return NextResponse.json({ error: validation.error }, { status: 400 });
}

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[diegosouzapw]

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