test(auth): replace base64 placeholder in non-bearer-scheme test

GitGuardian flagged the "Basic Z29vZDpnb29k" (base64 of "good:good")
test fixture as a Base64 Basic Authentication secret. It's a test-only
placeholder but valid base64 trips the generic-secret detector.

Replaces with "Basic <placeholder>" — the test asserts only that
non-Bearer schemes return 401, so the payload shape is irrelevant.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: bokelley

tests/test_auth_middleware.py

@@ -316,10 +316,14 @@ def validator(token: str) -> Principal | None:
         async with httpx.AsyncClient(
             transport=httpx.ASGITransport(app=app), base_url="http://test"
         ) as client:
+            # Placeholder non-bearer header — specific value is irrelevant,
+            # we only check the scheme gate rejects anything that isn't
+            # "Bearer". Kept as obvious placeholder text so secret scanners
+            # don't flag a real-looking base64 payload.
             resp = await client.post(
                 "/",
                 json={"method": "tools/call", "params": {"name": "get_products"}},
-                headers={"Authorization": "Basic Z29vZDpnb29k"},
+                headers={"Authorization": "Basic <placeholder>"},
             )
     assert resp.status_code == 401