fix(front): keep custom agent when starting new conversation from convo

[gabhubert]

Description

Fixes a regression from #31961: starting a new conversation with a custom agent from inside an existing conversation (mention dropdown / agent details / Cmd+K) navigated to /conversation/new?agent=<custom>, but the remounted homepage input bar defaulted to @dust and that selection was then mirrored back into the URL.

• useHandleMentions: skip draft/sticky/@dust defaults on the new-conversation page while ?agent= is present, so we don't race useAgentFromSearchParam
• useAgentFromSearchParam: when entering /conversation/new from an existing conversation, clear the applied marker and re-push the URL agent; only treat the URL agent as applied once the composer selection actually matches (avoids mirroring a stale default into the URL)
• AgentDetailsButtonBar: navigate to the new conversation directly instead of onClose() + push (double navigation race)

Before click: analyst details in conversation
After: new conversation with analyst selected
custom-agent-new-convo-fix.mp4

Tests

• Updated unit tests in useAgentFromSearchParam.test.ts, including:
  • re-push matching URL agent when entering a new conversation
  • do not mirror a default overwrite while the URL agent is still pending
• Manual: from an existing conversation, opened analyst details → New conversation → landed on /conversation/new?agent=analyst with @analyst selected (not @dust)

Risk

Low. Scoped to new-conversation agent selection / URL sync. Rollback is a straight revert.

Deploy Plan

Standard deploy.

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Slack Thread

  

[Fraggle]

Lgtm !