[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).

management/server/agentnetwork_budgetrule_realstack_test.go

@@ -102,11 +102,20 @@ func TestAgentNetwork_UpdateSettings_PreservesImmutableAndTogglesCollection(t *t
 	require.NotEmpty(t, before.Subdomain, "subdomain pinned at bootstrap")
 	assert.False(t, before.EnablePromptCollection, "prompt collection defaults off")
 
-	// Attempt to flip toggles AND smuggle a different cluster/subdomain — the
-	// immutable fields must be ignored.
+	// A cluster different from the one pinned at bootstrap must be rejected
+	// outright — never silently swapped or ignored.
+	_, err = mgr.UpdateSettings(ctx, adminUserID, &agenttypes.Settings{
+		AccountID:           accountID,
+		Cluster:             "attacker.cluster",
+		EnableLogCollection: true,
+	})
+	require.Error(t, err, "UpdateSettings with a mismatched cluster must fail")
+
+	// Flipping the toggles works with the pinned cluster echoed back (and
+	// with it omitted); the subdomain is never taken from the request.
 	updated, err := mgr.UpdateSettings(ctx, adminUserID, &agenttypes.Settings{
 		AccountID:              accountID,
-		Cluster:                "attacker.cluster",
+		Cluster:                clusterAddr,
 		Subdomain:              "evil",
 		EnableLogCollection:    true,
 		EnablePromptCollection: true,