refactor(provider): extract helpers from normalizeMessages

[szzhoujiarui-sketch]

• Extract shouldKeepContentPart, filterEmptyContentParts for Anthropic/Bedrock
• Extract scrubToolCallIds for Claude and Mistral tool call ID scrubbing
• Extract addDeepseekReasoning for Deepseek reasoning injection
• Extract extractInterleavedReasoning for interleaved models
• Extract sanitizeMessage and sanitizeToolResultOutput for message cleanup
• Remove TODO comment, add step-by-step comments in main function
• Net reduction: 34 lines (182 insertions, 216 deletions)

Issue for this PR

Closes #

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Refactors the normalizeMessages function to improve readability and maintainability.

• Extracts 6 helper functions for provider-specific logic
• Removes duplicate Anthropic/Bedrock empty content filtering code
• Adds step-by-step comments documenting the transformation pipeline
• Net reduction: 34 lines (182 insertions, 216 deletions)
• No behavioral changes, all existing tests pass

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How did you verify your code works?

• Ran all existing tests: bun test test/provider/transform.test.ts → 259 pass, 0 fail
• Typecheck passes: bun typecheck
• No behavioral changes verified by test results

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Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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[szzhoujiarui-sketch]

Refactors the normalizeMessages function in packages/opencode/src/provider/transform.ts to improve readability and maintainability.

• Extracts 6 helper functions for provider-specific logic
• Removes duplicate Anthropic/Bedrock empty content filtering code
• Adds step-by-step comments documenting the transformation pipeline
• Net reduction: 34 lines (182 insertions, 216 deletions)
• No behavioral changes, all existing tests pass
• Ran all existing tests: bun test test/provider/transform.test.ts → 259 pass, 0 fail
• Typecheck passes: bun typecheck
• No behavioral changes verified by test results

[github-actions]

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