[codex] Isolate phone call router test imports

[tianmind-studio]

Summary

• Stub the phone-call router's database, quota, auth, and lightweight form-parser import surfaces inside test_phone_calls.py.
• Keep the test focused on the phone router behavior without requiring Firestore, Redis/BYOK, or python-multipart in a lightweight Windows unit-test venv.
• Preserve real utils / database package paths so real modules such as utils.twilio_service can still import when needed.

Root cause

test_phone_calls.py already stubs Twilio and Firebase, but importing routers.phone_calls still pulled in real database/quota/auth dependencies. On this Windows lightweight backend venv, standalone collection failed before any phone-call assertion ran:

• python -m pytest backend\tests\unit\test_phone_calls.py --collect-only -q --tb=short -> ImportError: cannot import name 'firestore' from 'google.cloud'

After the import stubs, the TwiML tests advanced to Starlette form parsing, where the same lightweight venv lacked python-multipart. The test only sends URL-encoded fields, so this PR provides the minimum parser surface Starlette needs for those requests.

Validation

• python -m pytest backend\tests\unit\test_phone_calls.py --collect-only -q --tb=short -> 18 tests collected
• python -m pytest backend\tests\unit\test_phone_calls.py -q --tb=short -> 18 passed, 1 warning
• python -m pytest backend\tests\unit\test_action_item_date_validation.py backend\tests\unit\test_phone_calls.py -q --tb=short -> 44 passed, 1 warning
• python -m black --check --line-length 120 --skip-string-normalization backend\tests\unit\test_phone_calls.py
• python -m py_compile backend\tests\unit\test_phone_calls.py
• git diff --check
• scripts/pre-commit