[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.

management/internals/modules/agentnetwork/manager.go

@@ -86,12 +86,17 @@ type Manager interface {
 
 // PolicySelectionInput is the per-request selection envelope. The
 // proxy populates it from CapturedData (account, user, groups) plus
-// the provider llm_router resolved.
+// the provider llm_router resolved and the model it extracted.
 type PolicySelectionInput struct {
 	AccountID  string
 	UserID     string
 	GroupIDs   []string
 	ProviderID string
+	// Model is the already-normalised upstream model id the proxy extracted
+	// (parser strips Bedrock region/version, Vertex @version), so a
+	// case-insensitive compare suffices. Empty = undetermined → not permitted
+	// (fail closed).
+	Model string
 }
 
 // PolicySelectionResult names the policy that "pays" for this request