feat(web): add custom provider support for embedding & vision models, and fix memory_get Windows path bug!

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

plugins/cow_cli/cow_cli.py

@@ -1334,8 +1334,19 @@ class CowCliPlugin(Plugin):
                 return "linkai (legacy)", "text-embedding-3-small", 1536
             return "(legacy)", None, None
 
-        meta = EMBEDDING_VENDORS.get(provider_key) or {}
+        # Since we have added support for custom providers for vector models, this part should be modified accordingly:
+        # Custom providers ("custom:<id>") resolve to the "custom" vendor key.
+        resolved_key = "custom" if provider_key.startswith("custom:") else provider_key
+        meta = EMBEDDING_VENDORS.get(resolved_key) or {}
         model = cfg_model or meta.get("default_model")
+        # Custom provider model fallback: read from custom_providers entry.
+        if not model and provider_key.startswith("custom:"):
+            from models.custom_provider import parse_custom_bot_type, get_custom_providers, _find_provider_by_id
+            _, custom_id = parse_custom_bot_type(provider_key)
+            if custom_id:
+                entry = _find_provider_by_id(get_custom_providers(), custom_id)
+                if entry and entry.get("model"):
+                    model = entry["model"]
         dim = cfg_dim if cfg_dim > 0 else meta.get("default_dimensions")
         return provider_key, model, dim