Add auto-import retry to getCompletionsAtPosition in API

[auvred]

Fixes #64132

This PR adds a retry with (*Session).GetSnapshotWithAutoImports similar to

TypeScript/tsc/internal/lsp/server.go

Lines 1379 to 1391 in 6ed67b6

	if errors.Is(lsErr, ls.ErrNeedsAutoImports) {
	languageService, lsErr = s.session.GetLanguageServiceWithAutoImports(ctx, snapshot, params.TextDocumentURI())
	if lsErr != nil {
	return lsErr
	}
	if ctx.Err() != nil {
	return ctx.Err()
	}
	resp, lsErr = fn(s, ctx, languageService, params)
	if errors.Is(lsErr, ls.ErrNeedsAutoImports) {
	panic(info.Method + " returned ErrNeedsAutoImports even after enabling auto imports")
	}
	}

[Copilot]

🟡 Changes recommended

The retry path must propagate cancellation after auto-import snapshot preparation.

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

Pull request overview

Adds API completion retry support when auto-import data is required.

Changes:

• Retries completions using an auto-import-prepared snapshot.
• Generalizes language-service setup for alternate snapshots.
• Adds an auto-import completion regression test.

File summaries

File	Description
tsc/internal/api/session.go	Implements snapshot preparation and completion retry.
tsc/internal/api/session_completion_test.go	Verifies auto-import completions are returned.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Balanced

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