[Server][Schema] Implement SEP-1577: Sampling With Tools

[felixweinberger]

This is a tracking issue for implementation of SEP-1577.

Summary

This extension adds tools and toolChoice parameters to sampling/createMessage, enabling MCP servers to run their own agentic loops using the client's tokens under user supervision. The PHP SDK needs to implement:

• New tools and toolChoice parameters in CreateMessageRequest
• New content types: ToolUseContent and ToolResultContent in SamplingMessage
• Updated CreateMessageResult with array content support and explicit stopReason values
• New client capability: sampling.tools
• Soft-deprecation of includeContext (now fenced by sampling.context capability)

This reduces complexity for client implementations while unlocking agentic tool-calling patterns in sampling requests.

Related Issues & PRs

• Implementation PRs: n/a
• Related PRs: SEP-1577 - Sampling With Tools modelcontextprotocol#1796
• Related Issues: n/a