fix: validate openai-compatible base urls

[HwangJohn]

Summary

• validate provider base URLs before model construction
• reject OpenAI-compatible URLs that point at /chat/completions instead of the API root
• surface the same warning in credential diagnostics and interactive setup

Fixes #326.

Testing

• corepack pnpm exec prettier --check src/constants.ts src/agent/index.ts src/credentials.tsx src/env.ts test/constants.test.ts test/env-behavior.test.ts
• corepack pnpm run typecheck
• corepack pnpm exec vitest run test/constants.test.ts test/env-behavior.test.ts (fails only on the existing Windows chmod assertion in test/env-behavior.test.ts > writes the env file with 0600 permissions; the new base URL tests pass)

[Colin Francis (colifran)]

Nice one! Thanks HwangJohn!

[Colin Francis (colifran)]

HwangJohn - can you take a look at the failing lint check and the failing test here?

[HwangJohn]

Thanks for the heads up. I pushed a follow-up that merges the latest main, resolves the conflicts, and fixes the env-behavior test to use the dynamically imported env module.

Locally verified with:

• corepack pnpm run lint:check
• corepack pnpm run typecheck
• corepack pnpm exec vitest run test/env-behavior.test.ts -t "flags an OpenAI-compatible chat completions endpoint as a base URL warning"
• corepack pnpm exec vitest run test/constants.test.ts test/model-resolution.test.ts