Share dispatcher queue cancellation mechanics

[George Ng (GeorgeNgMsft)]

This PR removes duplicated dispatcher cancellation code while keeping the public cancellation APIs separate. cancelCommand and structured cancelAction still provide different ownership checks and result shapes, but they now share one internal helper for removing queued requests, marking running requests cancelled, and aborting their controller.

Changes

• Add an internal cancelQueuedRequest helper for request-ID-based queue cancellation.
• Reuse it from Dispatcher.cancelCommand.
• Reuse it from structured operation cancellation after the structured terminal result has been published.
• Preserve event-before-abort ordering, queued/running/not-found results, and idempotent running cancellation.
• Leave cancelCommandByClientId and the agent-RPC cancelAction(actionContextId) path unchanged.

Validation

• Dispatcher cancellation and queue suites: 5 suites, 112 tests.
• Dispatcher RPC: 19 tests.
• Agent-server structured host: 7 tests.
• Agent-server dependency build and formatting checks.
• Lint, complexity, circular-dependency, and test-debt ratchets against PR [2/3] Add direct action call path to dispatcher #2992 head a6049a3bf15966d09db5c0b5fc8e8b2deb0bf257.