feat(mcp): add full _meta field support for CallToolResult objects

[bostdiek]

• Extract and preserve complete _meta field from MCP CallToolResult responses
• Merge metadata into additional_properties of converted content items
• Handle isError field for proper error state integration
• Support arbitrary metadata like token usage, costs, and performance metrics
• Maintain backward compatibility with existing tool execution workflows
• Add comprehensive test coverage for all metadata scenarios including edge cases
• Update documentation with metadata handling examples and patterns

Fixes protocol compliance violation where _meta fields were being dropped,
enables proper monitoring and cost tracking of MCP tool usage.

Motivation and Context

Description

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.