[management] Check a provider's url and credential before saving it (#7301)

A bad upstream or key saved cleanly and surfaced minutes later as a failed
request or an empty model picker, with nothing pointing back at the record.

CreateProvider now spends the credential once against the vendor's model
listing. UpdateProvider does the same when the upstream, the key, the catalog
provider or the skip-TLS flag changed — only then, so renames and price edits
neither wait on a vendor nor fail because one is down. Both run before the store
write, so a rejected rotation leaves the working key where it was.

What cannot be checked still saves: no listing endpoint, no derivable Bedrock
control-plane host, a private upstream, a record skipping TLS verification.
Everything else blocks, outages included — 5xx, 429 and timeouts leave the
record unverified just as a refusal does. Refusals return 422 and carry no
status code or echoed URL.

Discovery now reads as a partial edit, so a retyped URL can be listed against
without also rotating the credential. Entries with their own listing host
(Bedrock) get their configured upstream resolved separately, since a successful
listing said nothing about it.

management/server/agentnetwork_realstack_test.go

@@ -101,10 +101,14 @@ func TestAgentNetwork_ProviderCRUD_FansOutToProxyAndClientPeers(t *testing.T) {
 	drain(proxyCh)
 
 	provider, err := agentMgr.CreateProvider(ctx, adminUserID, &agenttypes.Provider{
-		AccountID:   accountID,
-		ProviderID:  "openai_api",
-		Name:        "openai-test",
-		UpstreamURL: "https://api.openai.com",
+		AccountID:  accountID,
+		ProviderID: "openai_api",
+		Name:       "openai-test",
+		// A private address: the save-time credential check leaves it
+		// unchecked rather than spending a dummy key against the real
+		// api.openai.com, which the vendor refuses and which would make
+		// this test depend on the runner having egress.
+		UpstreamURL: "https://10.255.255.1",
 		APIKey:      "sk-test-key",
 		Enabled:     true,
 		Models:      []agenttypes.ProviderModel{{ID: "gpt-5.4"}},