feat(ai-proxy): add Zod validation for route bodies

[Scra3]

Summary

• Add schemas.ts with Zod schemas for runtime route validation
• Validate dispatchBody (ai-query route) - requires messages array with at least one message
• Validate invokeRemoteToolBody - requires inputs array with at least one message
• Validate invokeToolQuery - requires tool-name parameter
• Use permissive validation with .passthrough() - validates structure, lets OpenAI SDK handle field details
• Throw AIUnprocessableError with clear messages on validation failure

Files

• New: packages/ai-proxy/src/schemas.ts - Zod schemas
• New: packages/ai-proxy/test/schemas.test.ts - 12 unit tests
• Modified: packages/ai-proxy/src/router.ts - Integrate validation
• Modified: packages/ai-proxy/package.json - Add zod dependency

Test plan

• [x] All unit tests pass (yarn workspace @forestadmin/ai-proxy test)
• [x] Lint passes (yarn workspace @forestadmin/ai-proxy lint)
• [x] Build passes (yarn workspace @forestadmin/ai-proxy build)

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