[Deprecated-1b] Implement deprecated Sampling proxy support

[lucarlig]

Part of #5677

Split from #5690.

Spec: https://modelcontextprotocol.io/specification/2026-07-28/client/sampling
SEP: https://modelcontextprotocol.io/seps/2577-deprecate-roots-sampling-and-logging
Depends on: #5682 for the generic Multi Round-Trip Requests (MRTR) transport and retry flow.

Context

The Rust dataplane does not currently advertise or proxy Sampling. Sampling is deprecated in MCP 2026-07-28; this work is a deliberate compatibility exception for existing counterparts during the deprecation window. It must not add legacy protocol or session behavior, and the gateway must not become an LLM provider.

Gateway work

• Carry the per-request ClientCapabilities.sampling declaration and its tools/context sub-capabilities through the modern request path.
• Proxy sampling/createMessage as an MRTR inputRequest, and route the corresponding inputResponses entry back to the originating upstream request.
• Preserve opaque MRTR request state and deterministic retry behavior from [Major-4] Multi Round-Trip Requests (MRTR) and required resultType #5682.
• Enforce capability gates for tool use and context inclusion.
• Normalize deprecated includeContext: "thisServer" and "allServers" to omission or "none" when proxying.
• Emit a safe deprecation warning when Sampling is negotiated or used; do not log prompts, sampled content, model preferences, or tool payloads.
• Document migration to direct LLM-provider integration.
• Do not implement legacy server-initiated request/session behavior.

Acceptance criteria

• Modern client ↔ gateway ↔ modern upstream Sampling MRTR round-trips work end to end.
• Sampling tool fields are rejected unless sampling.tools is declared.
• Deprecated includeContext values are normalized and covered by tests.
• Deprecation warning, migration guidance, and removal note are documented.
• Focused tests cover capability gating, successful retry, declined/error responses, tools, and malformed sampling payloads.