feat(mcp): introduce unified request context API and enhance documentation

- Add StreamableMcpAnnotations2IT integration test covering:
  - Tool execution with progress notifications
  - Sampling and elicitation capabilities
  - Structured output support with CallToolResult
  - Resource and prompt handling
  - Completion functionality
  - Logging notifications
  - Client-server bidirectional communication

- Refactor documentation to use McpSyncRequestContext/McpAsyncRequestContext:
  - Replace McpSyncServerExchange/McpAsyncServerExchange examples
  - Add convenient helper methods (info(), progress(), ping(), elicit(), sample())
  - Update examples to show unified context API usage
  - Document capability checking (elicitEnabled(), sampleEnabled(), rootsEnabled())

- Add method filtering policies documentation:
  - Explain sync vs async method filtering behavior
  - Document stateful vs stateless filtering rules
  - Provide best practices for method filtering

- Improve special parameters documentation:
  - Update to show unified request context usage patterns
  - Add examples of accessing progress token via context
  - Document limitations of stateless servers
  - Show capability-aware implementation patterns
  - Note that McpSyncRequestContext/McpAsyncRequestContext work with both stateful and stateless modes

The unified context API provides a cleaner, more intuitive interface for MCP operations
and works seamlessly with both stateful and stateless server configurations.

Signed-off-by: Christian Tzolov <[email protected]>

Author: tzolov