feat(compare): enhance API signature comparison and display

[AmiteK23]

Description

Adds backend API signature comparison support to the stamp context compare command.

This PR:

• compares interface.apiSignature in diff results
• detects parameter changes, including added, removed, and modified parameters with type information
• detects return type, request type, and response type changes
• updates compare output to display detailed API signature deltas
• extends unit tests to cover API signature comparison scenarios

This completes comparison support for extracted backend API contract data.

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Type of Change

• Bug fix
• Feature
• Breaking change
• Docs
• Refactor
• CI / Tests
• Performance
• Other (please describe):

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Architectural Impact

• No contract/output changes
• Contract/output changed (describe below)

If contract/output changed, explain the impact on:

• Bundle structure: No bundle schema changes
• CLI flags: No CLI flag changes
• Watch/strict behavior: No watch/strict behavior changes
• Comparison output: apiSignature is now included in compare diff detection and detailed display output

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Testing

• Tested locally
• Tests added/updated
• All tests pass

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Related Issues

None.

Checklist

• Target branch is main
• Self-reviewed
• Docs updated (if needed)
• Changelog updated (if needed)

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Additional Notes

• Includes detailed parameter-level diff rendering for backend API signature changes
• Includes apiSignature in non-hash change detection for git baseline comparison mode
• Improves backend contract drift detection in compare results