feat: expose provider capability bounds to app server clients (#20049)

follow up of #19442. The app server now exposes provider-derived bounds
through a new v2 `modelProvider/read` method. The response reports the
configured provider map key as `modelProvider` and returns the effective
capability booleans so clients can align their UI with the same
provider-owned limits used by core.

codex-rs/app-server-protocol/schema/json/v2/ModelProviderCapabilitiesReadParams.json

@@ -0,0 +1,5 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "ModelProviderCapabilitiesReadParams",
+  "type": "object"
+}