feat(deepseek): add reasoning support

[njbrake]

Description

The base openai _normalize_reasoning_on_message already handles Deepseek, all we needed to do is flip it on and add the test

PR Type

🆕 New Feature ## Relevant issues

Checklist

• I have added unit tests that prove my fix/feature works
• New and existing tests pass locally
• Documentation was updated where necessary
• I have read and followed the contribution guidelines```

[njbrake]

Integration test https://github.com/mozilla-ai/any-llm/actions/runs/18564678032

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/any_llm/providers/deepseek/deepseek.py	100.00% <100.00%> (ø)

... and 33 files with indirect coverage changes

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