fix(server): serve draft-version requests statelessly per SEP-2567

[alexhancock]

Motivation and Context

Per modelcontextprotocol/modelcontextprotocol#2567 protocol-level sessions (and the Mcp-Session-Id header) are removed from the Streamable HTTP transport as of the draft protocol version (2026-07-28); servers speaking that version MUST serve requests statelessly.

The rust-sdk server assigned an Mcp-Session-Id in its default stateful_mode even when the negotiated protocol version was 2026-07-28. This gates the session code path on the negotiated version: sessions are used only for pre-draft versions (< 2026-07-28), and draft-version requests are served statelessly even in stateful_mode. Older versions retain full session behavior.

How Has This Been Tested?

No dedicated SEP-2567 conformance scenario exists (the conformance PR that would add one was closed unmerged); per the maintainer, every existing draft (2026-07-28) scenario already sends sessionless requests. Verified against the existing draft server suite:

cargo build -p mcp-conformance
STATELESS= ./target/debug/conformance-server &   # default stateful_mode
npx -y @modelcontextprotocol/conformance@0.2.0-alpha.9 server --url http://127.0.0.1:8001/mcp --suite draft -o results

The default (stateful) server now produces results byte-for-byte identical to an explicit stateless server (48 passed / 31 failed; the 31 remaining failures are pre-existing SEP-2575 server/discover gaps, out of scope). rmcp lib unit tests (232) and streamable-HTTP integration tests (session store, stale session, protocol version, JSON response — 19) pass locally.

Breaking Changes

None. The session code path is unchanged for all pre-2026-07-28 versions; only the draft version changes behavior, bringing it into spec compliance.

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

SEP-2575 (also in progress) removes the initialize handshake for the draft version. Once that lands, the version detection here can be simplified to a lifecycle check (an initialize request implies the pre-draft/stateful lifecycle; draft requests never carry initialize).

[DaleSeo]

Since this is a breaking change, the commit message should contains ! to pass CI like refactor!: rename stateful_mode to legacy_session_mode.