[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/auth/tunnel_cache_test.go

@@ -169,3 +169,32 @@ func TestTunnelCache_BoundedSizeEvictsOldest(t *testing.T) {
 	assert.NotNil(t, cache.get(keys[1]), "second-newest must remain cached")
 	assert.NotNil(t, cache.get(keys[2]), "newest must remain cached")
 }
+
+func TestTunnelCacheTTLFromEnv(t *testing.T) {
+	t.Run("unset uses default", func(t *testing.T) {
+		t.Setenv(envTunnelCacheTTL, "")
+		assert.Equal(t, tunnelCacheTTL, tunnelCacheTTLFromEnv())
+	})
+	t.Run("valid duration overrides", func(t *testing.T) {
+		t.Setenv(envTunnelCacheTTL, "45s")
+		assert.Equal(t, 45*time.Second, tunnelCacheTTLFromEnv())
+	})
+	t.Run("whitespace trimmed", func(t *testing.T) {
+		t.Setenv(envTunnelCacheTTL, "  2m ")
+		assert.Equal(t, 2*time.Minute, tunnelCacheTTLFromEnv())
+	})
+	t.Run("unparseable uses default", func(t *testing.T) {
+		t.Setenv(envTunnelCacheTTL, "nonsense")
+		assert.Equal(t, tunnelCacheTTL, tunnelCacheTTLFromEnv())
+	})
+	t.Run("non-positive uses default", func(t *testing.T) {
+		t.Setenv(envTunnelCacheTTL, "0s")
+		assert.Equal(t, tunnelCacheTTL, tunnelCacheTTLFromEnv())
+		t.Setenv(envTunnelCacheTTL, "-30s")
+		assert.Equal(t, tunnelCacheTTL, tunnelCacheTTLFromEnv())
+	})
+	t.Run("constructor honors override", func(t *testing.T) {
+		t.Setenv(envTunnelCacheTTL, "90s")
+		assert.Equal(t, 90*time.Second, newTunnelValidationCache().ttl)
+	})
+}