fix(delegation): add edge-case tests and SSRF caveat docstring

Red-team QA pass found no bugs but two test gaps:

- Whitespace-only api_key now tested (falls through to placeholder)
- IPv6 loopback [::1] now tested (already worked, just untested)
- 172.32.x boundary now tested (correctly NOT treated as private)
- Docstring note about private-network SSRF caveat: placeholder key
  gets 401'd by real auth servers, fail-fast by design

tools/delegate_tool.py

@@ -2106,6 +2106,15 @@ def _is_local_base_url(base_url: Optional[str]) -> bool:
       - localhost / loopback (127.0.0.1, ::1)
       - .local mDNS hostnames (e.g. studio.local)
       - RFC 1918 private networks (10/8, 172.16/12, 192.168/16)
+
+    .. note::
+        Any URL that resolves as "local" by this function will receive a
+        placeholder ``"ollama"`` API key.  If an internal service on a
+        private network actually requires authentication (e.g. a corporate
+        AI gateway at 192.168.x.x), the placeholder key will be rejected
+        by that server (401/403).  This is intentional — local servers that
+        genuinely don't need auth work out-of-the-box, while misconfigured
+        endpoints fail loudly rather than silently.
     """
     if not base_url:
         return False