fix(calendar): trust operator CA bundle in CalDAV test_connection (#4796)

* fix(calendar): trust operator CA bundle in CalDAV test_connection

The pre-flight test used httpx with trust_env=False, which ignored
SSL_CERT_FILE/REQUESTS_CA_BUNDLE. Self-signed CalDAV servers that
the real sync accepts (via caldav lib → requests → honors bundle)
were rejected by the test with CERTIFICATE_VERIFY_FAILED.

Build an explicit SSL context that loads the operator's CA bundle
and clears VERIFY_X509_STRICT (which rejects certs without a
keyUsage extension — common in self-signed setups). SSRF guards
(follow_redirects=False, trust_env=False) are preserved.

Fixes #4795
Fixes #4779

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(calendar): add regression tests and edge case handling for SSL context

Per review: add route-level regression tests covering SSL_CERT_FILE
precedence, VERIFY_X509_STRICT clearing, missing bundle graceful
fallback, and empty env var handling. Also log a warning when the
configured CA bundle path doesn't exist instead of silently falling
back to system CAs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* test(calendar): rewrite SSL tests to exercise route handler directly

Addresses review feedback: tests now use FastAPI TestClient to hit the
actual test_connection route, capturing the verify= kwarg passed to
httpx.AsyncClient. This ensures the route's SSL context construction
is covered, not a test-side duplicate.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* ci: retrigger CI (redirect hardening test is a CI-env flake, passes locally)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix(tests): remove module-level sys.modules stubs that leaked into other tests

The collection-time MagicMock stub of `caldav` replaced the real library
for every later test in the same process — test_caldav_redirect_hardening's
DAVClient became a mock that never sent the PROPFIND, failing its
must-reach-the-public-server assertion in CI. conftest already pre-imports
the real sqlalchemy/core.database, and the route's lazy imports are patched
per-request, so the stub block was both harmful and unnecessary.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* test(calendar): verify exact CA bundle precedence

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Alexandre Teixeira <alexandremagteixeira@gmail.com>

routes/calendar_routes.py

@@ -913,7 +913,24 @@ def setup_calendar_routes() -> APIRouter:
             '</d:prop></d:propfind>'
         )
         try:
-            async with httpx.AsyncClient(timeout=8.0, follow_redirects=False, trust_env=False) as cx:
+            # Build an SSL context that trusts the operator's custom CA bundle
+            # (SSL_CERT_FILE / REQUESTS_CA_BUNDLE) so self-signed CalDAV servers
+            # pass the pre-flight the same way they pass the real sync.
+            # trust_env=False is kept to block proxy/auth env leakage; the CA
+            # bundle is loaded explicitly instead.
+            import ssl as _ssl
+            _ssl_ctx = _ssl.create_default_context()
+            # Disable VERIFY_X509_STRICT so certs without a keyUsage extension
+            # (common in self-signed setups) are accepted, matching the
+            # requests/urllib3 behavior used by the CalDAV sync path.
+            _ssl_ctx.verify_flags &= ~_ssl.VERIFY_X509_STRICT
+            _ca_bundle = _os.environ.get("SSL_CERT_FILE") or _os.environ.get("REQUESTS_CA_BUNDLE")
+            if _ca_bundle:
+                if _os.path.isfile(_ca_bundle):
+                    _ssl_ctx.load_verify_locations(_ca_bundle)
+                else:
+                    logger.warning("CalDAV test: CA bundle %s not found, using system CAs", _ca_bundle)
+            async with httpx.AsyncClient(timeout=8.0, follow_redirects=False, trust_env=False, verify=_ssl_ctx) as cx:
                 r = await cx.request(
                     "PROPFIND", url,
                     auth=(user, pw),