feat: add prefer_structured_content parameter for response format con…

[Edison-A-N]

Add structured content response support

Add parameter to choose between structured JSON and text content responses. Defaults to False for backward compatibility.

Describe your changes

• Added prefer_structured_content parameter to FastApiMCP class
• When enabled, returns raw JSON response instead of formatted text content
• Maintains backward compatibility by defaulting to False
• Updated return type annotations to support both text and structured content
• Added comprehensive tests for structured content functionality

Issue ticket number and link (if applicable)

Closes #219 - Feature Request: Add Structured Content Support

Screenshots of the feature / bugfix

N/A

Checklist before requesting a review

• [x] Added relevant tests
• [x] Run ruff & mypy
• [x] All tests pass