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/conftest.py

@@ -0,0 +1,15 @@
+"""Shared test configuration — sets env vars ONCE before any test module runs.
+
+The env must be consistent for the whole pytest session: server.py freezes
+COLLECTION_NAME/QDRANT_URL at import time, while the integration fixture
+reads os.environ at fixture time. If any test module mutates os.environ after
+that (test_server.py used to set COLLECTION_NAME='test'), the fixture creates
+a different collection than the one server functions query → 404.
+Keep all env wiring here so import order no longer matters.
+"""
+
+import os
+
+os.environ.setdefault("QDRANT_URL", "http://localhost:6333")
+os.environ.setdefault("COLLECTION_NAME", "test_mcp_maildir")
+os.environ.setdefault("EMBEDDING_MODEL_NAME", "BAAI/bge-small-en-v1.5")