[codex] Isolate conversation count FieldFilter import

[tianmind-studio]

Summary

• Add a local FieldFilter fallback in test_conversations_count.py when google.cloud.firestore_v1 is unavailable.
• Keep using the real Firestore FieldFilter when the dependency is installed.
• Cover the case where another lightweight test leaves an empty google.cloud.firestore_v1 stub in sys.modules before this file is collected.

Why

This test validates query-building logic using only the field_path and value attributes of FieldFilter. On a lightweight Windows backend environment, collection failed before any assertions ran because google.cloud.firestore_v1 was not installed. Running after another stub-heavy test can also produce an import-name failure from an empty stub module. The fallback keeps this unit test self-contained without changing production code.

Validation

• python -m pytest backend\tests\unit\test_conversations_count.py --collect-only -q --tb=short
• python -m pytest backend\tests\unit\test_conversations_count.py -q --tb=short
• python -m pytest backend\tests\unit\test_action_item_date_validation.py backend\tests\unit\test_conversations_count.py -q --tb=short
• python -m pytest backend\tests\unit\test_conversations_count.py backend\tests\unit\test_action_item_date_validation.py -q --tb=short
• python -m black --line-length 120 --skip-string-normalization backend\tests\unit\test_conversations_count.py
• python -m py_compile backend\tests\unit\test_conversations_count.py
• git diff --check
• scripts\pre-commit