Prevent focusing in chat on page load

Related to #575660 (closed)

The MR fixes the regression, introduced in Moved responsibility of focusing in Duo Chat's ... (!214692 - merged), where Duo Chat was constantly stealing focus from other elements. The MR ensures that Duo Chat's input gets focus only when user switches to either new, or active mode of the Duo Chat. Those actions don't happen automatically and are always initiated by the user.

Before	After
	focus-fixed

Complex interactions

Classic	Agentic
fixed-classic-complex	fixed-agentic-complex

Some specific scenarios identified during the reviews

Interaction with the Active thread	Switching from Agentic to Classic
fixed-agentic-active-thread	fixed-agentic-to-classic