fix(providers): detect rejected reasoning history

[SulimanAbdulrazzaq]

Summary

Closes #220.

Auto-config now probes whether a thinking-capable provider accepts an assistant history message containing reasoning. A real non-2xx response sets sendReasoningInMessages=false; transport failures remain inconclusive and preserve the default. ProviderModal applies the explicit rejection while leaving reasoning_effort and other thinking settings unchanged.

Validation:

• src/server/providers/auto-config.test.ts: 17 passed
• ProviderModal.test.tsx: 13 passed with a dependency-safe targeted configuration
• Server and E2E TypeScript checks passed
• ESLint and Prettier passed
• git diff --check
• Full suite not run; the standard browser setup is blocked by an existing junctioned dependency path

AI-Enhanced Development

• AI Models: GPT-5.6 (Codex)

Cache Impact

• No