Support tool callbacks in MCP sampling

[EronWright]

Summary

Closes the tool callbacks functional gap in MCP sampling support — a follow-up to #2815, addressing one of the remaining items from #2809.

When an MCP server includes a tools array in a sampling/createMessage request, the host now drives its model with those tools and returns any tool_use blocks back to the server as ToolUseContent. The server remains responsible for executing the tool and continuing the loop in a follow-up sampling request (see sequence: #2815 (comment)).

What's new

• New SamplingWithToolsHandler type and SampleableWithTools interface — additive, parallel to the existing SamplingHandler / Sampleable. No breaking changes to the basic sampling path merged in feat(mcp): add sampling/createMessage support #2815.
• MCP toolset wires both handler types. At Initialize, exactly one of the SDK's mutually exclusive ClientOptions.CreateMessage* fields is populated — prefer with-tools when registered, fall back to basic.
• Capability handshake advertises sampling.tools so servers know the host can receive tool-enabled requests.
• Runtime handler (pkg/runtime/sampling.go):
  • Converts V2 multi-block messages: text, image/audio, tool_use → assistant ToolCalls, tool_result → MessageRoleTool rows (parallel tool_results expand to multiple chat.Message rows).
  • Converts []*mcp.Tool → []tools.Tool with a no-op handler (the server, not the host, executes).
  • Drives model.CreateChatCompletionStream, aggregates streamed tool calls.
  • Builds result Content with TextContent + ToolUseContent blocks; stopReason: \"toolUse\" when tool calls are present.
• New limits: maxSamplingTools=64, maxSamplingToolCalls=32.

Out of scope (separate gaps from #2809)

• Human-in-the-loop approval UI
• Model-preference hints

Test plan

• Unit tests pass: go test ./pkg/runtime/... ./pkg/tools/...
• go build ./... clean
• go vet ./... clean
• task lint (0 offenses)
• gofmt clean on all changed files
• End-to-end with a minimal MCP server that calls ServerSession.CreateMessageWithTools:
  • Handshake includes sampling.tools capability
  • LLM receives the server-supplied tools
  • Response contains ToolUseContent with stopReason: \"toolUse\" when the model emits a tool_use
  • Follow-up sampling request with tool_result blocks is converted correctly and the loop terminates with endTurn