1. Embedding model: support custom provider
- Add "custom" entry to EMBEDDING_VENDORS with supports_dim_param=False
- Parse custom:<id> credentials and model fallback in agent_initializer
- Expand custom_providers as custom:<id> entries in Web UI dropdown
2. Vision model: support custom provider
- Add custom:<id> routing in _route_by_provider_id
- Add _build_custom_provider reading credentials from custom_providers
- Expand custom_providers in Web UI dropdown, add validation in _set_vision
3. Fix memory_get Windows path validation bug!
- str.startswith(path+'/') always False on Windows due to backslashes, So All Users can not use "memory_get" tool in Windows.
- Use os.path.realpath + os.sep, consistent with MemoryService
4. Fix historical needsModel:false bug preventing embedding provider switch
- Change embedding needsModel to true in console.js
- Support custom:<id> resolution in cow_cli /memory status, also for adding custom provider support
Closes#2908, Closes#2880
