FU-P13-T7: Enforce strict structuredContent compliance for empty-content tool results

[SoundBlaster]

Description

Adds targeted regression tests and troubleshooting documentation for the strict structuredContent compliance requirement on empty-content tools/call responses (FU-P13-T7).

The core injection logic (structuredContent: {} for result.content: []) was already implemented in transform.py (P4-T1/BUG-T5). This PR adds:

• 6 new regression tests in TestEmptyContentStrictCompliance covering the isError=true + content:[] path, notification passthrough, and already-compliant response preservation
• A new troubleshooting section for strict MCP clients (Cursor/Codex) encountering the "empty result" variant of the structuredContent schema violation

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactoring
• CI/CD improvement

Quality Gates

• make test - All tests pass with ≥90% coverage (471 passed, 5 skipped; 95.6% coverage)
• make lint - No linting errors (ruff check src/ clean)
• make format - Code is properly formatted
• make typecheck - Type checking passes
• make doccheck - Documentation is synced with DocC (if docs changed) — DocC catalog not present; N/A

Documentation Sync

docs/ file	DocC file
docs/troubleshooting.md	mcpbridge-wrapper.docc/Troubleshooting.md

• Documentation changes are synced with DocC catalog (or N/A) — DocC catalog not present in repo

Testing

• Added/updated tests for new functionality (6 new tests in TestEmptyContentStrictCompliance)
• All tests pass locally (471 passed, 5 skipped)
• Manually tested the changes

Checklist

• Code follows the project's style guidelines
• Self-review completed
• Comments added for complex code
• Documentation updated (if needed)
• No new warnings generated
• PR title is descriptive