language-server: avoid noisy errors for expected git check-ignore exits

[AhmmedSamier]

Motivation

• git check-ignore can exit with code 1 to indicate "no matches", but the indexer treated any non-zero exit as an error which produced noisy console.error output during incremental/batch indexing and benchmarks.

Description

• Suppress console.error for git check-ignore when the exit code is 1 while preserving error logs for other failures in WorkspaceIndexer.processGitCheckBatch (language-server/src/core/workspace-indexer.ts).
• Add a small helper WorkspaceIndexer.getGitExitCode to reliably extract a git exit code from both structured error objects and error messages.
• Continue to resolve all queued git-check promises as non-ignored so indexing proceeds deterministically when git check-ignore returns no matches.

Testing

• Ran unit tests with cd language-server && bun test src/core/workspace-indexer.test.ts, and all tests passed (9 pass, 0 fail).
• Executed the incremental benchmark with cd language-server && bun run benchmarks/incremental.bench.ts, which completed and showed expected timings while reducing noisy check-ignore error messages for exit code 1 (bench output recorded).
• Verified the change only affects logging behavior around check-ignore handling and does not alter how checks resolve during indexing.

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Codex Task