fix(gateway): fingerprint full auth token in agent cache signature (#3247)

Previously _agent_config_signature() used only the first 8 characters of
the API key, which causes false cache hits for JWT/OAuth tokens that share
a common prefix (e.g. 'eyJhbGci'). This led to cross-account cache
collisions when switching OAuth accounts in multi-user gateway deployments.

Replace the 8-char prefix with a SHA-256 hash of the full key so the
signature is unique per credential while keeping secrets out of the
cache key.

Salvaged from PR #3117 by EmpireOperating.

Co-authored-by: EmpireOperating <EmpireOperating@users.noreply.github.com>

gateway/run.py

@@ -4684,10 +4684,18 @@ class GatewayRunner:
         prompt cache hits.
         """
         import hashlib, json as _j
+
+        # Fingerprint the FULL credential string instead of using a short
+        # prefix. OAuth/JWT-style tokens frequently share a common prefix
+        # (e.g. "eyJhbGci"), which can cause false cache hits across auth
+        # switches if only the first few characters are considered.
+        _api_key = str(runtime.get("api_key", "") or "")
+        _api_key_fingerprint = hashlib.sha256(_api_key.encode()).hexdigest() if _api_key else ""
+
         blob = _j.dumps(
             [
                 model,
-                runtime.get("api_key", "")[:8],  # first 8 chars only
+                _api_key_fingerprint,
                 runtime.get("base_url", ""),
                 runtime.get("provider", ""),
                 runtime.get("api_mode", ""),