Add metaParams support to MCP tool calls and expose MCP response _meta in
MCPToolArtifact. This allows callers to pass metadata parameters to MCP
tools and access response metadata returned by MCP tool calls alongside
structured_content.

Per the MCP specification (https://modelcontextprotocol.io/specification/2025-06-18/basic/index#general-fields),
_meta is defined as a separate field at the request level. This implementation correctly treats metaParams
as a distinct parameter passed to session.call_tool(meta=...), following the spec's CallToolRequest structure
where _meta is a sibling to 'name' and 'arguments', not nested within arguments. Metadata is passed exclusively
through interceptors, keeping tool arguments clean and protocol-level metadata separate from LLM-facing parameters.

Changes:
- Bump mcp dependency from 1.9.2 to 1.22.0
- Add metaParams field to MCPToolCallRequest for passing metadata to MCP servers
- Add metaParams parameter to tool calls passed as 'meta' to session.call_tool()
- Add _meta field to MCPToolArtifact (with total=False to make both fields optional)
- Update tools and interceptors to support metadata parameters
- Add comprehensive tests for metadata passing and interceptor composition
- Update README documentation with interceptor-based metadata examples
- Ensure clean separation: tool arguments remain pure, metadata handled via interceptors