test: dedup smoke fixtures + register shared config plugin (#2066)

[yastman]

This pull request was created by @kiro-agent on behalf of @yastman 👻

Comment with /kiro fix to address specific feedback or /kiro all to address everything.
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Parent: #1515 (test audit), categories D3 + D4.

#2066 calls out three issues with smoke test fixtures:

• D3: tests/smoke/conftest.py declared its own redis_url, duplicating tests/fixtures/config.py::redis_url with a different scope.
• D4: tests/smoke/conftest.py read QDRANT_URL/QDRANT_COLLECTION env vars directly, with QDRANT_COLLECTION default gdrive_documents_bge diverging from tests/fixtures/config.py default test_documents.

Root cause: tests/fixtures/config.py existed as a "shared fixtures" file but was never registered with pytest, so its session-scoped fixtures were not actually visible to any test tier. Each tier rolled its own.

Fix

1. tests/conftest.py — declare

   pytest_plugins = ["tests.fixtures.config"]

   so the shared fixtures resolve repo-wide.

2. tests/fixtures/config.py — change qdrant_collection default from "test_documents" to "gdrive_documents_bge" to match telegram_bot/config.py and compose.yml. Smoke tests against the default config now hit the same collection name the bot uses end-to-end.

3. tests/smoke/conftest.py — drop the local redis_url fixture; the shared session-scoped one now resolves automatically. The require_live_services / qdrant_service / cache_service / voyage_service fixtures continue to work; they consumed redis_url via request.getfixturevalue(), which keeps working.

Contract test

tests/contract/test_smoke_fixtures_dedup_contract.py with three invariants:

• shared plugin is registered from root conftest;
• qdrant_collection default == production contract value;
• tests/smoke/conftest.py does not redefine any shared fixture.

Verification

uv run pytest tests/contract/test_smoke_fixtures_dedup_contract.py -v
  3 passed

uv run pytest tests/smoke/ --collect-only -q
  73 tests collected (no fixture-resolution errors)

Closes #2066.