Fix Output Schema validation issue in tool result processing

[rakdutta]

Issue #1328

This PR enhances how tool output schemas are managed, validated, and displayed. It adds support for optional schemas, structured result validation, and UI improvements for easier schema editing and review.

Backend:

• Added _extract_and_validate_structured_content for schema-based result validation using jsonschema.
• Updated tool logic to store output_schema as None when not provided.
• Added structured_content to ToolResult for validated data and improved logging for schema changes.

Frontend:

• Enhanced admin UI to handle optional schemas, manage read-only states, and display validation errors clearly.
• Updated placeholder text to better reflect the expected schema format.

Verification:
Verified tool behavior in MCP Inspector for both cases — with and without output schema. Functionality works as expected.