fix: Mark completions as incomplete when local completions block autoimport

[AryanBagade]

Summary

When local completions exist (e.g., UsersController), autoimport completions are skipped for performance. However, the response was marked as is_incomplete=false, causing clients (Zed, nvim) to cache results and filter locally instead of requesting new completions from the server.

This meant when typing UsersM, the client would filter cached results and never show UsersManager from autoimport.

Now we mark is_incomplete=true when local completions exist, so clients request completions again as the user types more characters, allowing autoimport completions to appear when local ones no longer match.

Fixes #1398

Test Plan

• Added two new integration tests:
  • test_completion_incomplete_with_local_completions_blocking_autoimport
  • test_completion_autoimport_shown_when_local_no_longer_matches
• Ran test.py - all tests pass including conformance tests

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[meta-codesync]

@kinto0 has imported this pull request. If you are a Meta employee, you can view this in D90121759.

[stroxler]

Review automatically exported from Phabricator review in Meta.

[meta-codesync]

@kinto0 merged this pull request in 9909406.