feat: runtime provider configuration API (BYOK setup), rebased on main

Rebased onto upstream main (post cloud-video-gen merge) as a single clean
commit; adapts to the new provider-kind matrix (LiterLLM, LocalAsr,
LocalImageGen, LocalVideoGen, CloudVideoGen dialect) and the
associated-function server style.

- Engine::build_provider extracted from startup assembly; shared with
  Engine::add_provider_config, which registers a provider at runtime behind
  an RwLock (replacing same-name entries); find_provider, routing_providers,
  and discover_all all consult the runtime list, so a registered provider
  is routable and discoverable immediately
- SDK: GET/POST /v1/config/providers with masked listing derived from live
  capability cards; add validates (name/kind/api_key), appends a
  [[providers]] entry to the resolved config file for restart survival,
  then registers + refreshes. ProviderConfig::provider_kind made pub so
  external API layers can validate before calling into the engine.
- EngineConfig::resolved_path: MOFA_ENGINE_CONFIG env > ./config.toml >
  ~/.config/mofa-engine/config.toml.
- Tests: factory smoke over the full kind matrix + full runtime registration
  (discoverability, invoke routing, disabled rejection) against a live mock
  endpoint.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: amos-aios

.DS_Store

@@ -0,0 +1,9 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(git diff *)",
+      "Read(//tmp/**)",
+      "Bash(cargo check *)"
+    ]
+  }
+}