fix(sessions): tighten gateway model-run key predicate

The model-run prune predicate fell back to testing the raw sessionKey
when parseAgentSessionKey returned null, so unscoped keys like
`explicit:model-run-<uuid>` and shapes with empty agent ids were
eligible for the new default 24h cleanup. Restrict matching to keys
that successfully parse as agent-scoped with a non-empty agent id,
and add negative tests covering unscoped, empty-agent, extra-segment,
and whitespace-padded keys.

Refs #88632 (review feedback before merge).

src/config/sessions/store-maintenance.ts

@@ -190,8 +190,18 @@ export function shouldRunSessionEntryMaintenance(params: {
 }
 
 export function isGatewayModelRunSessionKey(sessionKey: string): boolean {
+  if (
+    !/^agent:[^:]+:explicit:model-run-[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i.test(
+      sessionKey,
+    )
+  ) {
+    return false;
+  }
   const parsed = parseAgentSessionKey(sessionKey);
-  const rest = normalizeLowercaseStringOrEmpty(parsed?.rest ?? sessionKey);
+  if (!parsed || !parsed.agentId.trim()) {
+    return false;
+  }
+  const rest = normalizeLowercaseStringOrEmpty(parsed.rest);
   return /^explicit:model-run-[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/.test(
     rest,
   );

src/config/sessions/store.pruning.test.ts

@@ -278,6 +278,58 @@ describe("pruneStaleModelRunEntries", () => {
       ),
     ).toBe(false);
   });
+
+  it("rejects non-canonical session keys that do not parse as agent-scoped", () => {
+    // Unscoped: missing `agent:<id>:` prefix — parseAgentSessionKey returns null.
+    expect(
+      isGatewayModelRunSessionKey("explicit:model-run-123e4567-e89b-12d3-a456-426614174000"),
+    ).toBe(false);
+    // Empty agent id segment: not a canonical `agent:<id>:` scoped key.
+    expect(
+      isGatewayModelRunSessionKey("agent::explicit:model-run-123e4567-e89b-12d3-a456-426614174000"),
+    ).toBe(false);
+    // Extra colon segment between agent id and `explicit:` — rest starts
+    // with `extra:` and fails the predicate's regex.
+    expect(
+      isGatewayModelRunSessionKey(
+        "agent:main:extra:explicit:model-run-123e4567-e89b-12d3-a456-426614174000",
+      ),
+    ).toBe(false);
+    // Whitespace-padded keys are non-canonical even though parseAgentSessionKey
+    // trims before normalizing; the predicate intentionally checks the original
+    // key shape before accepting a model-run key.
+    expect(
+      isGatewayModelRunSessionKey(
+        "  agent:main:explicit:model-run-123e4567-e89b-12d3-a456-426614174000",
+      ),
+    ).toBe(false);
+    expect(
+      isGatewayModelRunSessionKey(
+        "agent:main:explicit:model-run-123e4567-e89b-12d3-a456-426614174000  ",
+      ),
+    ).toBe(false);
+  });
+
+  it("matches canonical keys whose agent id begins with model-run-", () => {
+    // Guards against an over-tight fix that confuses the agent id segment
+    // with the `explicit:model-run-<uuid>` rest segment.
+    expect(
+      isGatewayModelRunSessionKey(
+        "agent:model-run-foo:explicit:model-run-123e4567-e89b-12d3-a456-426614174000",
+      ),
+    ).toBe(true);
+  });
+
+  it("preserves case-insensitive matching for canonical keys", () => {
+    // normalizeLowercaseStringOrEmpty + parseAgentSessionKey's normalization
+    // lower-case everything outside opaque peer IDs, so a mixed-case
+    // canonical key still matches.
+    expect(
+      isGatewayModelRunSessionKey(
+        "agent:Main:Explicit:Model-Run-123E4567-E89B-12D3-A456-426614174000",
+      ),
+    ).toBe(true);
+  });
 });
 
 describe("capEntryCount", () => {