feat(api): add project AI chat endpoint with validation

[flooryyyy]

Motivation

• Provide an in-project AI chat endpoint to let project users send a short conversation and receive an assistant reply with usage metadata.
• Ensure inputs are validated and bounded to prevent excessive upstream usage and request payload abuse.

Description

• Add POST /api/projects/[id]/ai/chat implemented with projectAction, parsing and validating messages via Zod and returning an assistant message plus usage metadata.
• Enforce size limits including content-length check, serialized body byte-size (256KB), per-message char limit (8,000), max messages (40), and overall char cap (120,000), returning 413 when exceeded.
• Introduce an AI provider abstraction in src/app/lib/ai/provider.ts with generateProjectAssistantReply and an OpenAI-compatible implementation driven by AI_CHAT_ENDPOINT, AI_CHAT_API_KEY, AI_CHAT_MODEL, and AI_CHAT_PROVIDER, with upstream error mapping and logging.

Testing

• Ran static checks via npm run check which includes eslint and tsc and completed successfully.
• Ran the test suite (vitest run) which completed with all tests passing (Test Files 11 passed, Tests 55 passed).

---

Codex Task