fix: persist modelLabel to DB and derive from spec as fallback

All chat (including bedrock specs) goes through /api/agents/chat/:endpoint.
The buildOptions middleware nests conversation settings inside model_parameters,
then the AgentClient constructor was never given modelLabel — so getSaveOptions()
always returned modelLabel: undefined, which was stripped by removeNullishValues.
The field was never saved to the database, so every page refresh lost it.

Two fixes:

1. api/server/services/Endpoints/agents/initialize.js (backend — the real bug)
   Pass modelLabel from model_parameters to the AgentClient constructor, so it
   gets persisted to the conversation in MongoDB.

2. client/src/hooks/Conversations/useGetSender.ts (frontend — defensive fallback)
   When a loaded conversation has a spec but no modelLabel (older conversations
   that were saved before this fix), derive the display label from the spec
   configuration instead of falling back to "AWS Bedrock".

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

api/server/services/Endpoints/agents/initialize.js

@@ -441,6 +441,7 @@ const initializeClient = async ({ req, res, signal, endpointOption }) => {
     endpointType: endpointOption.endpointType,
     resendFiles: primaryConfig.resendFiles ?? true,
     maxContextTokens: primaryConfig.maxContextTokens,
+    modelLabel: endpointOption.model_parameters?.modelLabel,
     endpoint: isEphemeralAgentId(primaryConfig.id) ? primaryConfig.endpoint : EModelEndpoint.agents,
   });