Diagnostics: Tighten DP086 for async assemble contracts

[Skymly]

Parent

#324

What to build

Tighten DP086 so Step Builder assemble-contract diagnostics describe the Phase 2 rules: a valid [BuilderAssemble] returns product T or Task<T> / ValueTask<T>; illegal shapes (bare Task/ValueTask, duplicate assemble, more than one CancellationToken, etc.) stay DP086 with an actionable message. No new diagnostic IDs.

Acceptance criteria

• DP086 message/description mention sync product T and async Task<T> / ValueTask<T> as valid assemble returns
• DP086 guidance remains actionable for illegal assemble contracts called out in Spec Spec: Step Builder async assemble / BuildAsync (Phase 2) #324
• Descriptor/catalog tests updated if they assert DP086 text
• No new DP### allocated; Runtime / SourceGenerators behavior unchanged in this PR
• Single-module PR: Diagnostics only; PR references Closes this issue and Relates to Spec: Step Builder async assemble / BuildAsync (Phase 2) #324

Blocked by

None — can start immediately.