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_integration.py

@@ -1,13 +1,14 @@
-"""Integration tests for mcp-maildir with a real Qdrant instance."""
+"""Integration tests for mcp-maildir with a real Qdrant instance.
+
+Env vars (QDRANT_URL, COLLECTION_NAME, EMBEDDING_MODEL_NAME) are set in
+tests/conftest.py — don't set them here, they must match server.py's
+import-time snapshot for every module in the session.
+"""
 
 import os
 import uuid
 import pytest
 
-os.environ["QDRANT_URL"] = "http://localhost:6333"
-os.environ["COLLECTION_NAME"] = "test_mcp_maildir"
-os.environ["EMBEDDING_MODEL_NAME"] = "BAAI/bge-small-en-v1.5"
-
 from server import get_qdrant_client, get_embedding_model, search_emails, read_email
 from qdrant_client.http import models