fix: propagate validation errors (#7787)

fix: validate MCP configuration in model config

Fixes #7334

The Validate() function was not checking if MCP configuration
(mcp.stdio and mcp.remote) contains valid JSON. This caused
malformed JSON with missing commas to be silently accepted.

Changes:
- Add MCP configuration validation to ModelConfig.Validate()
- Properly report validation errors instead of discarding them
- Add test cases for valid and invalid MCP configurations

The fix ensures that malformed JSON in MCP config sections
will now be caught and reported during validation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Signed-off-by: majiayu000 <1835304752@qq.com>
Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>