feat: add SEP-2575 discovery and SEP-2322 MRTR models

[erenatas]

Introduce the first scoped 2026-07-28 stateless MCP protocol surface in
rmcp without changing the default protocol version or existing stateful
flows.

Changes:

• Add server/discover request and DiscoverResult models for SEP-2575.
• Wire discover through ServerHandler with a default implementation derived from get_info().
• Add 2026-07-28 protocol-version awareness while keeping LATEST at 2025-11-25.
• Add Multi Round-Trip Request models for SEP-2322:
  • ResultType
  • InputRequest
  • InputRequiredResult
  • InputResponse
  • inputResponses/requestState on CallToolRequestParams
• Allow CallToolResult to carry optional resultType for wire compatibility.
• Route server/discover through CustomRequest and typed new results through CustomResult to avoid adding variants to public exhaustive enums.
• Preserve PartialEq on existing task payload params.
• Document why MRTR still exposes SEP-2577-deprecated sampling variants.
• Add deserialization and round-trip coverage for discover, inputRequired, typed input responses, and protocol-version behavior.
• Update schemars snapshots for MRTR request fields and CallToolResult.resultType

Motivation and Context

This adds the first scoped rmcp model-layer support for the MCP 2026-07-28 release candidate stateless protocol work.

The change is tied primarily to:

• SEP-2575: server/discover as the stateless replacement path for handshake-time capability discovery.
• SEP-2322: Multi Round-Trip Requests, including inputRequired, inputRequests, inputResponses, and requestState.
• Partial SEP-2260 alignment: adds the wire/model foundation for stateless server-to-client request flows, without enforcing runtime lifecycle constraints yet.

This is needed so rmcp can begin supporting fully stateless MCP servers while preserving the current 2025-11-25 default behavior.

How Has This Been Tested?

Tested locally in rmcp with:

• cargo fmt
• cargo test -p rmcp --test test_deserialization -- --nocapture
• cargo test -p rmcp
• git diff --check

Also tested against a real remote MCP server locally by temporarily pointing its workspace rmcp dependency to this local SDK branch.

Breaking Changes

No intentional breaking changes.

Existing 2025 initialization/session flows remain available, and ProtocolVersion::LATEST remains 2025-11-25.

Validation against a server surfaced downstream compatibility concerns from rmcp 2.0 renames, so this PR also adds deprecated aliases for older names:

• Content
• PromptMessageContent
• PromptMessageRole

Users should not need to update code for this PR, but they may see deprecation warnings if they still use those old names.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

This PR is intentionally scoped to the model/handler foundation for SEP-2575 server/discover and SEP-2322 MRTR. It does not implement stateless HTTP transport mode, Mcp-Method/Mcp-Name validation, OAuth hardening, Tasks, Apps, or cache TTL
behavior.

MRTR includes sampling request/response variants because sampling is deprecated by SEP-2577 but still retained by the protocol lifecycle.