Decouple snapshot ownership from project.Session so api.Session only uses one in LSP mode

[Andrew Branch (andrewbranch)]

This is a refactor that will make #64154 cleaner. A project.Session owns a canonical/latest snapshot, which makes sense as the state backing an LSP server. But that will make it awkward to use in the standalone API when we want to remove the concept of a singular latest snapshot.¹ This moves the infrastructure necessary to clone a snapshot (mostly caches) into a project.SnapshotHost (open to naming suggestions) so api.Session can deal with that for most of its operations, while in LSP mode, the methods that can actually affect LSP state still go through project.Session, so they're very easy to identify.

¹ This doesn't yet remove the single-latest-snapshot concept from the API, as I think that will necessarily change how things look all the way down to the client, and I wanted to take the opportunity to extract this as a scoped refactor.

[Copilot]

🟡 Changes recommended

Async connection shutdown can wait indefinitely for handlers whose context remains live after EOF.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Decouples snapshot ownership from project.Session, enabling standalone API sessions to manage snapshots directly while LSP sessions retain canonical state.

Changes:

• Introduces SnapshotStore for snapshot creation, caches, IDs, and disposal.
• Splits API construction into standalone and LSP-backed sessions.
• Tracks asynchronous IPC handlers during connection shutdown.

File summaries

File	Description
tsc/internal/project/snapshotstore.go	Adds shared snapshot infrastructure.
tsc/internal/project/snapshot.go	Moves snapshot dependencies to SnapshotStore.
tsc/internal/project/snapshot_test.go	Updates snapshot adoption test.
tsc/internal/project/session.go	Delegates snapshot ownership to the store.
tsc/internal/project/refcountcache_test.go	Updates cache ownership references.
tsc/internal/project/extendedconfigcache_test.go	Updates extended-config cache references.
tsc/internal/project/api.go	Exposes background snapshot adoption.
tsc/internal/lsp/server.go	Creates LSP-backed API sessions.
tsc/internal/ipc/conn_async.go	Waits for asynchronous handlers on shutdown.
tsc/internal/ipc/conn_async_test.go	Tests handler waiting behavior.
tsc/internal/api/session.go	Supports standalone and LSP snapshot stores.
tsc/internal/api/session_temporary_test.go	Uses the LSP session constructor.
tsc/internal/api/session_createprogram_test.go	Updates and expands program tests.
tsc/internal/api/session_completion_test.go	Uses the LSP session constructor.
tsc/internal/api/session_apistate_test.go	Adds standalone-session coverage.
tsc/internal/api/server.go	Removes standalone project.Session creation.

Review details

• Files reviewed: 16/16 changed files
• Comments generated: 1
• Review effort level: Balanced

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[Andrew Branch (andrewbranch)]

This is now the standalone API’s “latest snapshot” instead of using s.projectSession.Snapshot(), and will be removed in a subsequent PR.

[Andrew Branch (andrewbranch)]

Drive-by fix

[Copilot]

🟡 Changes recommended

project.Session.Close currently recurses indefinitely, and one changed line also fails gofmt.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

• Files reviewed: 15/15 changed files
• Comments generated: 2
• Review effort level: Balanced

[Copilot]

🟡 Changes recommended

Direct host cloning drops the LSP client and can reuse content-mapped diagnostics under the wrong locale.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

• Files reviewed: 15/15 changed files
• Comments generated: 1
• Review effort level: Balanced