feat: add protocol version override support for client session initialization

[STiFLeR7]

Description

This PR adds support for client session protocol version overrides during initialization, enabling clients to negotiate older or custom protocol versions (e.g. 2024-11-05) with MCP servers.

Changes

• Updated ClientSession.initialize(...) to accept a custom protocol_version.
• Added protocol_version field to the high-level Client dataclass.
• Added protocol_version field to ClientSessionParameters in ClientSessionGroup.
• Implemented corresponding unit and integration tests.

[cubic-dev-ai]

No issues found across 8 files

_{Re-trigger cubic}

[STiFLeR7]

Confirmed and fixed in f55e795a. You're right: negotiate_auto's successful-discovery path (session.adopt(result); return) never consulted protocol_version — it was only threaded into the two initialize() fallback call sites. Traced the exact control flow to confirm before changing anything.

Went with your option 1: since the PR description's own stated intent is "enabling clients to negotiate older or custom protocol versions," an override has to actually win, not depend on whether the server happens to answer server/discover. So negotiate_auto now short-circuits at the top — if protocol_version is set, it skips the probe entirely and goes straight to the legacy handshake at that version. The two now-unreachable inline branches that used to pass protocol_version to the fallback initialize() calls collapsed back to their original unconditional form, since the override is always None by the time the loop runs.

Added:

• A unit test on negotiate_auto proving the probe is skipped even when the stub's discover script would otherwise return a valid modern result (test_a_protocol_version_override_skips_discovery_and_forces_the_legacy_handshake).
• An e2e test over a real streamable-HTTP server with mode="auto" (default) + override, asserting initialize is the only method sent and server/discover never is (test_auto_mode_with_a_protocol_version_override_skips_discover_and_initializes), plus its requirements-manifest entry.

Both fail on the pre-fix code and pass after (verified via git stash). Full suite, ruff, and pyright are clean; the only failures are 31 pre-existing ones unrelated to this file (confirmed identical with the fix stashed out).

Separate, smaller point from your review I did not change: the arbitrary-custom-version consistency gap (ClientSession.initialize() still rejects any server-echoed version outside HANDSHAKE_PROTOCOL_VERSIONS) is real but out of scope for this fix — happy to open a follow-up if that's wanted.

One more thing surfaced while testing this end-to-end: for an in-process Server/MCPServer target, Client(server, protocol_version_override=...) with the default mode="auto" currently hits a hard MCPError: Method not found rather than silently dropping the override — _connect_inproc's non-legacy connector wires up a DirectDispatcher peer (modern_on_request) that never implements the legacy initialize RPC at all, so negotiate_auto's new short-circuit has nothing to call. That's a separate, pre-existing connector-selection gap (mode picks the connector before protocol_version_override is known), unrelated to this bug and outside what I'm comfortable deciding unilaterally here. Flagging in case it's worth its own issue — happy to file one if useful.

[STiFLeR7]

Rebased onto main (89 commits) and fixed a genuine lint failure the rebase surfaced: RequestResponder and mcp.shared.session were removed upstream since this test was added. Updated message_handler to the current IncomingMessage type, matching every other handler in this file. Ruff, pyright, and all 92 tests in the file pass.