Run symbol-returning completion on the API checker

[Vorobey31415]

Set CheckerLifetimeAPI in handleGetCompletionsAtPosition so completion symbols resolve on the same checker GetTypeOfSymbol uses, fixing a nil-pointer panic (e.g. people. where people: string[]). Document the rule on setupLanguageService and add a regression test

[Copilot]

Pull request overview

This PR fixes a nil-pointer panic caused by completion entries returning symbol handles created on an ephemeral checker, which later became unresolvable when clients re-queried symbol types on the persistent API checker. It ensures symbol-producing completions run under CheckerLifetimeAPI, documents the lifecycle rule for setupLanguageService, and adds a regression test.

Changes:

• Documented how setupLanguageService’s internal checker acquisition interacts with symbol/type handle resolvability and the API checker lifetime.
• Updated handleGetCompletionsAtPosition to wrap the request context with CheckerLifetimeAPI when IncludeSymbol is requested.
• Added a regression test that requests completion symbols for string[] members and verifies their types can be resolved without panicking.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
internal/api/session.go	Pins symbol-returning completion requests to the API checker lifetime and documents when this is safe.
internal/api/session_completion_test.go	Adds regression coverage for resolving types of completion-provided symbols (prevents the nil-pointer crash).

[Vorobey31415]

@microsoft-github-policy-service agree company="JetBrains"

[andrewbranch]

Good catch, thanks!