fix: enable DeepSeek thinking on compat routes

[MackDing]

Summary

• enable DeepSeek thinking provider options through OpenAI-compatible routes when the model is a DeepSeek reasoning family model
• keep the old DeepSeekProvider behavior intact
• add regression tests for compat-routed DeepSeek reasoning models

Why

Issue #24 reports The reasoning_content in the thinking mode must be passed back to the API.
The agent loop only enabled providerOptions.deepseek.thinking for DeepSeekProvider + deepseek-reasoner, so DeepSeek reasoning models routed through the OpenAI-compatible provider path missed the required option.

Validation

• npm test

[MackDing]

Quick follow-up: I merged the latest main into this branch, resolved the conflict in src/core/agent.ts, and verified locally with npm run typecheck. From my side this should now be ready for maintainer review.