Diagnostics: Tighten DP086 for async assemble contracts

[Skymly]

Summary

• Tighten DP086 diagnostic copy so product type T and async Task/ValueTask returns are named correctly for Phase 2 assemble contracts.
• Update descriptor, Verify snapshot, and unit assertions to match Spec Diagnostics: Tighten DP086 for async assemble contracts #325.

Fixes #325

Test plan

• CI Ci / CiPack green
• Confirm DP086 void-assemble Verify snapshot and descriptor tests pass
• Spot-check message text names product T vs task transport clearly

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Note

Low Risk
Diagnostic message/description and test snapshot updates only; no generator logic, auth, or data-handling changes.

Overview
Updates DP086 (GenerateBuilderAssembleContractMismatch) message and description so valid [BuilderAssemble] returns are explicitly sync product T or async Task<T> / ValueTask<T>, and illegal cases (void, bare Task/ValueTask, duplicate assemble, multiple CancellationToken) are called out.

Also refreshes the void-assemble Verify snapshot, descriptor unit assertions, and changelog entry. No generator validation behavior changes in this PR.

^{Reviewed by Cursor Bugbot for commit 92d8324. Configure here.}

[cursor]

Stale comment

Autopilot status

Merge-ready for @Skymly — no action taken this pass.

Gate	Status
Conflicts	None (MERGEABLE)
Review threads	None open
Required CI	Green (build-test, build-test-net48, pack)
Bugbot	Pass

Note: branch is behind main (mergeStateStatus: BEHIND). Update from base before merge if branch protection requires an up-to-date head; otherwise safe to merge when you choose.

Leaving merge / auto-merge / ready-for-review to you.

  

_{Sent by Cursor Automation: DesignPatterns — Autopilot PR}

[cursor]

Stale comment

Autopilot status

Merge-ready for @Skymly — no code changes this pass.

Gate	Status
Conflicts	None (MERGEABLE)
Behind main	0 (2 commits ahead)
Review threads	None open
Required CI	Green (build-test, build-test-net48, pack)

Leaving merge to @Skymly.

  

_{Sent by Cursor Automation: DesignPatterns — Autopilot PR}

[cursor]

Autopilot status

Merge-ready for @Skymly — no further action this pass.

Gate	Status
Conflicts	None (MERGEABLE)
Base sync	Up to date with main (0 behind / 2 ahead)
Review threads	None open
Required CI	Green (build-test, build-test-net48, pack)

Leaving merge / auto-merge / ready-for-review to you.

  

_{Sent by Cursor Automation: DesignPatterns — Autopilot PR}