[management] Align agent-network API contracts for API clients (#7026)

## Describe your changes

Work on the Terraform provider (terraform-provider-netbird #177–#183)
surfaced places where the agent-network API broke its own contracts or
deviated from the conventions the rest of the management API follows,
forcing client-side workarounds.

Settings reads now follow the settings-endpoint convention: GET always
answers with a JSON object. Before bootstrap it returns the defaults
with an empty cluster/subdomain/endpoint (previously 200 with a JSON
`null` body, while the spec said 404). The settings PUT can bootstrap
the account by carrying a `cluster` — previously the row could only come
into existence through the first provider create, and a settings-first
setup was impossible; a differing cluster on a bootstrapped account is
rejected instead of silently ignored. PUT remains full-state.

The provider PUT schema promised omit-preserves semantics for several
operator-editable fields that the handler never delivered (it builds the
row from the request, like every other update handler). The schema
wording now matches the shipped full-state behavior; only the api_key
(secret) and session keys stay preserved by the manager. Identity
headers are always present in provider responses so an explicitly
cleared value round-trips as an empty string.

The Go REST client gains the full agent-network surface (catalog,
providers, policies, guardrails, budget rules, settings), including a
shim translating the legacy 200+`null` settings body from older servers
into an `IsNotFound` error.

Note for reviewers: the dashboard special-cased the `null` settings
body; it needs a small follow-up for the new defaults response (in
progress).

shared/management/client/rest/client.go

@@ -147,6 +147,10 @@ type Client struct {
 	// ReverseProxyTokens account-scoped proxy access tokens used to register
 	// self-hosted (bring-your-own-proxy) `netbird proxy` instances.
 	ReverseProxyTokens *ReverseProxyTokensAPI
+
+	// AgentNetwork NetBird Agent Network (AI/LLM gateway) APIs: catalog,
+	// providers, policies, guardrails, budget rules and account settings.
+	AgentNetwork *AgentNetworkAPI
 }
 
 // New initialize new Client instance using PAT token
@@ -209,6 +213,7 @@ func (c *Client) initialize() {
 	c.ReverseProxyClusters = &ReverseProxyClustersAPI{c}
 	c.ReverseProxyDomains = &ReverseProxyDomainsAPI{c}
 	c.ReverseProxyTokens = &ReverseProxyTokensAPI{c}
+	c.AgentNetwork = &AgentNetworkAPI{c}
 }
 
 // NewRequest creates and executes new management API request