fix(tasks): report cancelled fixture status consistently

[1fanwang]

Motivation and Context

The tasks-lifecycle report omitted its cancelled-status check when task creation or polling failed. It also accepted a completed status for the cancellable fixture. The check now requires that fixture's cancelled status and reports missing prerequisites as failures. MCP cancellation remains cooperative.

Closes #485.

How Has This Been Tested?

On Node 22.17.0, the lifecycle regressions drive the real runner against local HTTP task fixtures. They require SUCCESS for cancelled, FAILURE for completed/failed, and an untestable failure for missing tasks or polling timeouts.

Before: 7169291. After: this PR's head. The dependency manifests are identical.

npm ci --ignore-scripts --cache .work-cache/npm
# On the before checkout, bring in the regression test:
git restore --source=897bb9830f0ba0dcd59ad77934b293a99464412f -- src/scenarios/server/tasks/lifecycle.test.ts
# Run on both checkouts:
npm test -- src/scenarios/server/tasks/lifecycle.test.ts --reporter=verbose --maxWorkers=2

Raw log excerpts

Before, completed fixture:

AssertionError: expected [] to deeply equal [ ObjectContaining{…} ]
- [
-   ObjectContaining {
-     "details": {
-       "statusAfterCancel": "completed",
-     },
-     "errorMessage": "slow_compute fixture contract requires status:\"cancelled\" after cancellation while running; got \"completed\"",
-     "status": "FAILURE",
-   },
- ]
+ []

      Tests  5 failed (5)
exit_code=1

After, including both cancelled and completed assertions:

      Tests  5 passed (5)
exit_code=0

Breaking Changes

None. Missing prerequisites now appear as a failure row.

Types of changes

• Bug fix

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• Focused tests pass locally; full suite not run
• I have added appropriate error handling
• I have updated documentation as needed

Additional context

These fixtures are not an independent task-capable SDK.

AI disclosure: GitHub Copilot CLI wrote this change and its tests, and an automated coordinator reviewed the diff and test output before submission.

[pcarleton]

@panyam would you be willing to give this one a review?

[panyam]

Hey @pcarleton and @1fanwang very sorry about the late one. Went through it. Thanks heaps for this. Left a couple of (minor) comments.

Also added a live SDK run results. Branch merged onto 7169291, tasks-lifecycle run against three servers:

server	lifecycle	sep-2663-tasks-get-status-cancelled
C# SDK cbd8a58 (ConformanceServer, /stateless)	10/10 SUCCESS	SUCCESS, statusAfterCancel: "cancelled"
Go SDK 8075fb3 (conformance/everything-server, no tasks support)	9 emitted, 8 FAILURE	absent from the report
mcpkit f70e1488 (examples/tasks-v2)	10/10 SUCCESS	SUCCESS, statusAfterCancel: "cancelled"

Nice to see 2 independent implementations satisfy the new check, both going from 9 to 10 green. I could not find task support in the official Go SDK, and its run is what the inline note above is about (did I miss something).

Also, the branch is based on 74edef3, where the eight wire-schema-valid failures that main has since fixed are still live. So a verification run on it would look redder than it should be. Worth doing a rebase.

[panyam]

Thanks for the quick turn around @1fanwang - Ran it against the new head and both emit now. LGTM. @pcarleton good from my side.