Upgrade rmcp to 3.3.0 and activate cancellation regression

[lucarlig]

Modern Streamable HTTP cancellation could stall before the first response event. Upgrade rmcp from 3.2.0 to 3.3.0 and activate the existing regression that verifies cancelling a 2026-07-28 tool call reaches the backend.

Move downstream_cancellation_is_relayed_to_backend from future_contracts into the active gateway suite, preserving its three-second timeout. Remove the duplicate cancellation tests and stale ignore reasons, and update the testing wiki to reflect 12 remaining ignored tests. The regression uses modern discovery; it does not exercise a legacy client.

Refresh the lockfile with cargo update: update bitflags, cc, lru-slab, smallvec, tinyvec, and uuid, and remove the unused tinyvec_macros dependency.

Validation:

• Formatting and Clippy across the workspace.
• Focused cancellation regression and full workspace tests with all features.
• Production build with plugins.
• Wiki build and tests with mdBook 0.5.3.
• PR CI: nextest with all features, dependency advisories/bans/licenses, cargo-shear, benchmark compilation, secret scanning, and DCO.

[lucarlig]

/conformance