[management, proxy] scope agent-network model allowlist per policy/group and provider (#6905)

## Describe your changes

The model-allowlist guardrail was merged into one account-wide union and
enforced flat on every request, ignoring which policy/group/provider
authorised it. With multiple policies — especially a mix of guardrailed
and un-guardrailed ones — this caused:

- **false-allow**: a model allowlisted for one group/provider leaked to
any caller; and
- **false-deny**: an un-guardrailed policy (intended unrestricted) was
blocked by another policy's allowlist.

Enforcement is now policy/group-aware, mirroring `llm_limit_check`:

- **Management (`SelectPolicyForRequest`) is authoritative.** It uses
the request model (already carried in
`CheckLLMPolicyLimitsRequest.model`, previously ignored) to keep only
applicable policies whose guardrails permit the model; no
allowlist-enabled guardrail = unrestricted. Denies
`llm_policy.model_blocked` when policies govern the (provider, groups)
but none permits the model.
- **Proxy `llm_guardrail` becomes a per-provider fail-closed backstop.**
The synthesiser emits an allowlist only for providers every authorising
policy restricts; the middleware keys off the resolved provider id and
keeps unknown-model fail-closed.

proxy/internal/middleware/builtin/llm_request_parser/guardrail_allowlist_test.go

@@ -25,10 +25,17 @@ func runParserGuardrail(t *testing.T, url string, body []byte, allowlist []strin
 	})
 	require.NoError(t, err, "parser must not error")
 
-	guard := llm_guardrail.New(llm_guardrail.Config{ModelAllowlist: allowlist})
+	const providerID = "prov-under-test"
+	guard := llm_guardrail.New(llm_guardrail.Config{
+		ProviderAllowlists: map[string][]string{providerID: allowlist},
+	})
+	// The real chain has llm_router stamp the resolved provider id before the
+	// guardrail runs; the parser doesn't, so add it here so the guardrail can
+	// scope the allowlist to this provider.
+	meta := append([]middleware.KV{{Key: middleware.KeyLLMResolvedProviderID, Value: providerID}}, parsed.Metadata...)
 	out, err := guard.Invoke(context.Background(), &middleware.Input{
 		Slot:     middleware.SlotOnRequest,
-		Metadata: parsed.Metadata,
+		Metadata: meta,
 	})
 	require.NoError(t, err, "guardrail must not error")
 	require.NotNil(t, out, "guardrail must return an output")