feat: Replace preview session process spawn with lightweight config query

[charlesvien]

Problem

Creating a new task required spawning a full preview session just to fetch model/mode config options, adding too much latency + complexity.

I don't get the value of these anymore, if someone else does please let me know if I am way off here. We added preview sessions at a time before the gateway had a models API endpoint and before a ton of infrastructure we now have.

The worst part is the preview sessions don't help prompt submit -> agent working time because they are only used to fetch model and mode config options. It's not used as the session for the actual task on prompt submission and tbh that's probably for the best.

Closes #1347
Closes #1363

Changes

1. Add getPreviewConfigOptions method to AgentService that fetches config without spawning a session
2. Replace usePreviewSession hook with usePreviewConfig hook using a simple tRPC query
3. Remove preview session service infrastructure
4. Update TaskInputEditor and TutorialStep to pass config options as props instead of relying on session state
5. Refactor ReasoningLevelSelector and UnifiedModelSelector to accept options via props
6. Export additional gateway model helpers from agent package

How did you test this?

Manually

[charlesvien]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat: Replace preview session process spawn with lightweight config query #1438 👈 (View in Graphite)
• fix: Fix model selection lost during task creation #1437
• fix: Persist new task draft across navigation #1436
• main

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[adboio]

zero context on the history of the preview session but this seems reasonable, good stuff 🫡