feat(bedrock): completion reasoning

[njbrake]

Description

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 tests https://github.com/mozilla-ai/any-llm/actions/runs/18594959871

[codecov]

Codecov Report

❌ Patch coverage is 78.43137% with 11 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/any_llm/providers/bedrock/utils.py	78.00%	5 Missing and 6 partials ⚠️

Files with missing lines	Coverage Δ
src/any_llm/providers/bedrock/bedrock.py	90.82% <100.00%> (ø)
src/any_llm/providers/bedrock/utils.py	76.54% <78.00%> (-1.15%)	⬇️

... and 33 files with indirect coverage changes

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