🧩 chore: Extract Agent Client Utilities to /packages/api (#11789)

Extract 7 standalone utilities from api/server/controllers/agents/client.js
into packages/api/src/agents/client.ts for TypeScript support and to
declutter the 1400-line controller module:

- omitTitleOptions: Set of keys to exclude from title generation options
- payloadParser: Extracts model_parameters from request body for non-agent endpoints
- createTokenCounter: Factory for langchain-compatible token counting functions
- logToolError: Callback handler for agent tool execution errors
- findPrimaryAgentId: Resolves primary agent from suffixed parallel agent IDs
- createMultiAgentMapper: Message content processor that filters parallel agent
  output to primary agents and applies agent labels for handoff/multi-agent flows

Supporting changes:
- Add endpointOption and endpointType to RequestBody type (packages/api/src/types/http.ts)
  so payloadParser can access middleware-attached fields without type casts
- Add @typescript-eslint/no-unused-vars with underscore ignore patterns to the
  packages/api eslint config block, matching the convention used by client/ and
  data-provider/ blocks
- Update agent controller imports to consume the moved functions from @librechat/api
  and remove now-unused direct imports (logAxiosError, labelContentByAgent,
  getTokenCountForMessage)

packages/api/src/agents/index.ts

@@ -1,5 +1,6 @@
 export * from './avatars';
 export * from './chain';
+export * from './client';
 export * from './context';
 export * from './edges';
 export * from './handlers';