Redesign Duo Chat model selection behind new_model_selection flag

What does this MR do and why?

Redesigns the Duo Chat model selector behind the new_model_selection feature flag (disabled by default). The current dropdown shows provider, description, and cost on every row and sits in a header row above the input; the new design is a compact tertiary button next to the submit button, with details moved to a popover.

• New ChatModelSelectDropdown: name-only rows, Default badge, hover- and keyboard-focus-driven popover with description and cost, search
• Recent group (last 3 used models) persisted in localStorage
• The Agentic toggle is replaced by directional actions with identical visibility conditions: a "Use classic chat" button in the dropdown footer (standalone next to the submit button when the dropdown is unavailable), and a "Use agentic chat" button on the classic chat side
  • Considered placing "Use agentic chat" button in the same position as model selection, but would require bigger DOM restructuring in classic chat, may be follow up but minimal value given sunsetting of that mode
• Header row above the textarea is removed; closed button strips the gateway's " - " name suffix to save space
• Flag-off rendering is unchanged

Before	After	After - With non-agentic chat available	After - Classic chat mode

Relates to

How to test

1. Setup AI features in GDK
2. Enable :new_model_selection FF
3. Open a chat with Agentic mode toggled on (if Classic license, otherwise should always be on) and select a model