Return an API error when opened files cannot be loaded into a project

[Copilot]

Analysis

The crash reported on #64204 occurs when openFiles requests an unreadable virtual identity: ts-loader opens App.vue.ts, but its host serves App.vue. Snapshot construction proceeds without a containing project, then panics while building the response. This also reproduces with createSnapshot, not only update.

Fix

• Validation: Check project membership after ensuring each requested file; reject invalid opens through existing client-error and snapshot-cleanup paths. Keep the response invariant assertion.
• Coverage: Exercise sync/async creation, updates, successful retries, deleted-file reopens, and snapshot isolation.
• Contract: Document rejection when a file cannot be loaded into a project. Correctly served virtual files remain supported; ts-loader’s filename aliasing still needs correction.

Copilot Checklist

I successfully ran the applicable command at the end of my session, and it completed without error:

• npx hereby validate
• npx hereby validate --api (for TypeScript API changes)

[Copilot]

Copilot review overview

🟢 Approval recommended

The validation uses existing error cleanup paths and has comprehensive sync and async regression coverage.

Review effort: Balanced
Findings: None

What changed in this PR

Returns a client error instead of panicking when snapshot-opened files cannot be loaded into a project.

Changes:

• Validates project membership for opened files.
• Documents the failure contract.
• Adds sync and async regression coverage.

File	Description
tsc/​internal/​project/​projectcollectionbuilder.go	Rejects files without a project.
tsc/​internal/​api/​proto.go	Documents rejection behavior.
packages/​typescript/​test/​sync/​api.test.ts	Tests synchronous snapshot behavior.
packages/​typescript/​test/​async/​api.test.ts	Tests asynchronous snapshot behavior.
packages/​typescript/​src/​api/​proto.generated.ts	Exposes the documented API contract.

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[Andrew Branch (andrewbranch)]

In the future, it would be great to attach a reason to this error, but I'm afraid that might be a lot of plumbing the way things are set up right now.