Fix streamed WarpGrep result handling

[bhaktatejas922]

Closes #9.

Summary

• Await the final value from streamed WarpGrepClient.execute() before formatting the result.
• Add a regression test for promised final WarpGrep results so the plugin does not format the promise as a failed search.

Tests

• bun test
• bun run build
• bun run typecheck
• bun run ci
• Live local sanity check with warpgrep_codebase_search against this repo returned 1 context instead of the issue failure.