fix(rmcp): route peer cancellation by lifecycle

[aurokin]

Fixes #1251.

Implements the approach agreed to in #1251 (comment).

Problem

In legacy sessions, incoming server notifications/cancelled messages remove an outbound client waiter instead of cancelling the server-originated request's RequestContext.ct. This leaves the handler running and can affect an unrelated request with the same ID.

Removing outbound cleanup entirely would break modern subscription termination.

Fix

Route cancellation by the established lifecycle:

• Cancel the exact inbound token for legacy requests.
• Retire only an actual outbound subscriptions/listen waiter for modern server cancellation, independently of notification-channel registration.
• Reuse existing startup metadata so discovery retains modern semantics even when selecting an older application version.
• Keep string and numeric request IDs distinct.
• Suppress cancelled handler responses and errors during EOF draining while preserving uncancelled responses during graceful shutdown.

No dependency, wire-schema, or macro changes. Outgoing server teardown and same-direction ID reuse are outside this fix.

Regression coverage

The tests cover string-ID reverse cancellation on a live connection, handler-token observation, response suppression, subsequent traffic, legacy initialization, direct peers, unknown IDs, and equal IDs in opposite directions.

The subscription regression inspects the actual waiter after an ordered ping. It covers modern discovery selecting either application version, ordinary outbound request immunity, and listen requests without a registered notification channel.

Drain regressions cover successful and failed cancelled handlers and graceful completion of uncancelled handlers. Existing RoleServer stdio coverage remains in place.

Validation

The clean candidate passed:

• Targeted tests: 209 unit, 5 cancellation, 12 subscription, and 1 drain.
• Nightly formatting and focused Clippy.
• SemVer and public-API checks with default and non-local features.
• Full-range commitlint.

Full all-feature and non-local suites passed on the same production source before the final test-only error-response variant. That variant passed targeted validation, including against the published release with its original lockfile.

The same cancellation files also passed targeted validation after merging upstream 3023198 in a separate development branch. That merge is not included in this single-commit PR head. The regressions fail against unpatched upstream; an independent reproduction on 46db531 is recorded in #1251.

Full Clippy previously failed at an unchanged clippy::question_mark warning in auth.rs. The unchanged base reproduced the same warning with the same compiler and lockfile. No unrelated lint fix or suppression is included.