feat(protocol): implement MCP 2025-11-25 Phase 3 - Tool Calling in Sampling

Add complete sampling types with tool support:
- SamplingCapability with tools/context support
- CreateMessageRequestParam with tools, toolChoice, maxTokens
- CreateMessageResult with stop_reason
- ToolChoice and ToolChoiceMode (auto/required/none)
- Content::ToolUse for LLM tool invocation requests
- Content::ToolResult for returning tool execution results
- ToolResultContent for text/image results
- SamplingMessage, SamplingRole, SamplingContent
- ModelPreferences with hints and priorities
- ContextInclusion enum
- stop_reasons module with standard constants

Enables agentic workflows where servers request LLM sampling
with tools, and handle multi-turn tool use conversations.

Author: avrabe

integration-tests/src/end_to_end_scenarios.rs

@@ -182,7 +182,7 @@ impl McpBackend for E2ETestBackend {
                 logging: Some(LoggingCapability {
                     level: Some("debug".to_string()),
                 }),
-                sampling: Some(SamplingCapability {}),
+                sampling: Some(SamplingCapability::default()),
                 ..Default::default()
             },
             server_info: Implementation::new(format!("E2E Test Server: {}", self.name), "1.0.0"),