#### Fixes #51082

#### Description :

When a user configures `default_config_options` (e.g. a specific model)
for an ACP agent, the setting was only applied when creating a new
session. Resuming or loading an existing session (e.g. from history)
skipped this step, causing the model dropdown to revert to "Default
(recommended)".

#### Fix : 

Extract the config options defaulting logic into a shared helper
apply_default_config_options and call it consistently across all three
session entry points: new_session, resume_session, and load_session
Release Notes:

#### Release notes :
Fixed model dropdown resetting to "Default (recommended)" when opening a
previous conversation from history. Configured models (via
default_config_options) are now correctly applied when resuming or
loading existing sessions.


#### Video :
[Screencast from 2026-03-15
12-55-58.webm](https://github.com/user-attachments/assets/977747b9-390c-4f78-91fc-91feace444e1)