test: fix collection name mismatch — env wiring centralized in conftest

Root cause of the CI 404s: test modules mutated os.environ at import time
(test_server.py set COLLECTION_NAME='test', test_integration.py
'test_mcp_maildir'). With pytest's alphabetical import order,
server.COLLECTION_NAME froze at whichever value was set when server was
first imported, while the fixture read os.environ at runtime — often a
different name (log proof: fixture created 'test', server queried
'test_mcp_maildir' → 404 on all 6 tests).

conftest.py now sets QDRANT_URL/COLLECTION_NAME/EMBEDDING_MODEL_NAME once,
before any test module import; env mutation removed from test files.
Unit suite still green (43 passed, 6 deselected).

tests/test_server.py

@@ -1,8 +1,8 @@
-"""Unit tests for mcp-maildir server pure functions."""
+"""Unit tests for mcp-maildir server pure functions.
 
-import os
-os.environ["QDRANT_URL"] = "http://localhost:6333"
-os.environ["COLLECTION_NAME"] = "test"
+Env vars are set in tests/conftest.py (shared session-wide) — see the
+conftest docstring for why test modules must not touch os.environ.
+"""
 
 from server import (
     normalize_email_address,