feat: Add Langfuse governance trace exporter

[nagasatish007]

Adds tealtiger.integrations.langfuse module that exports governance decisions as Langfuse spans with appropriate levels (ERROR/WARNING/DEFAULT).

• LangfuseGovernanceExporter class with trace() callback
• Maps GovernanceDecision → Langfuse span with metadata
• DENY → ERROR level, MONITOR → WARNING, ALLOW → DEFAULT
• Includes tests and example
• Session-based trace reuse (same session = same parent trace)

Closes #321

Pull Request

Description

Provide a clear and concise description of what this PR does.

Fixes #(issue number)

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Code refactoring
• Test improvements

Changes Made

Testing

Describe the tests you ran to verify your changes:

• Unit tests pass (pytest)
• Type checking passes (mypy src)
• Linting passes (ruff check src tests)
• Manual testing performed
• Added new tests for new functionality

Checklist

• My code follows the project's code style
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published

Breaking Changes

If this PR introduces breaking changes, describe them here and provide migration instructions.

Screenshots (if applicable)

Add screenshots to help explain your changes.

Additional Notes

Add any other context about the PR here.

[nagasatish007]

good