AWS Transform: unit-test generation choice + status-only UT substeps

[Rajanna-Karthik]

What this adds

The language-server side of the unit-test generation experience for AWS Transform (.NET). It powers the Visual Studio behavior.

Customers choose up front whether unit tests get generated; this passes that choice to the transformation job and tells the IDE which steps to show as plain progress.

Changes (language server)

1. Carry the customer's unit-test choice to the job

• Passes the up-front "generate unit tests?" selection through to the transformation so it acts on the right decision.
• An unselected option is treated as a real "no"; a client that does not offer the option simply keeps today's behavior.

2. Mark unit-test steps as status-only

• Flags unit-test-generation substeps as status-only so the IDE renders them as plain progress (no checkpoint controls or "View Results"), like the other transformation substeps.

Notes

• Main merged into the branch cleanly (no conflicts).
• Backend contract is already shipped; CR-300232958 fixes a related backend issue where an up-front "no" could be overridden mid-job.
• Includes unit tests covering choice-forwarding (all input states) and status-only rendering.

[laileni-aws]

Reviewed together with aws-toolkit-visual-studio-staging#2927. No blocking defects in this diff itself; one question needs an answer before merge because it spans both PRs.

Question (potentially blocking): steps absent from checkpoint-settings.json

The toolkit side excludes status-only substeps from the checkpoints payload (it filters on ShowCheckpointCheckbox), so the UT substeps flagged here will never appear in checkpoint-settings.json. populateCheckpointsOnPlan then defaults them to HasCheckpoint = true for display. What does the orchestrator do with a step that is missing from the settings file in interactive mode? If it treats missing as "pause", the UT substeps will block the job with no UI to approve them. If so, either the toolkit must send an explicit false for status-only steps, or setCheckpoints should fill them in here.

Non-blocking

• isUnitTestGenerationStep matches the exact display name. In the test fixture the gen substep has the same name as its parent ("Generate Unit Tests"), so any nested children of gen (for example error-fix rounds) would also be flagged. Harmless today because Level 3 never shows toggles, but brittle if the service renames the step. Consider asking the service for a machine-readable label, or restricting the match to parents whose own parent is root.
• atxTransformHandler.ts L1174: (request.startTransformRequest as any).GenerateUnitTests — the field is now typed on StartTransformRequest in models.ts, so the cast can be dropped.

[mehishar]

I verified in orchestrator - status-only UT substeps are exempt from pausing so the toolkit omitting them from the payload has no bearing on whether they pause; the "hidden UI + missing key ⇒ stall" coupling doesn't actually exist. This is not a blocker and the current omit-based approach is safe.
Regarding the other non-blockers, we can take them up as follow up later