feat(task-input): preselect server-side team/user default model

[VojtechBartos]

Problem

PostHog now supports team-level and per-user default AI run preferences for task runs, resolved server-side (PostHog/posthog#70671, UI in PostHog/posthog#70683). PostHog Code ignores them: the task input picker preselects the device-local lastUsedModel, so a project-wide default never reaches runs started from the desktop app, and preferences don't follow the user across devices.

Changes

• PostHogAPIClient.getTaskRunDefaults() fetches GET /api/projects/:id/tasks/my_config/ and returns the resolved_ai_run_defaults triple (user preference over project default). Returns null on any failure, missing endpoint (older servers), or when nothing is configured - defaults are a nicety and must never block task creation.
• usePreviewConfig fetches the defaults alongside the gateway options. Preselection order for the model picker is now: server default (when it targets the current adapter and the gateway still offers the model) > device-local last-used model > gateway default. The default's reasoning effort is applied with it when the model supports it. Changing the picker still affects only the current composition, so a one-off pick never overrides the server-side preference.

Not in this PR (follow-ups): passing the server default through the one-click inbox flows (resolveDefaultModel / ReportModelResolver), and omitting the triple from cloud-run creation so the backend resolves it at run time (the saga currently must send runtime_adapter for initial_permission_mode; with this PR the picker already carries the server default into the run explicitly, and the backend applies defaults for any caller that omits them).

How did you test this?

• vitest packages/api-client/src/posthog-client.test.ts - 70 passed, including 3 new cases for getTaskRunDefaults (snake_case → client shape mapping, null on 404 from older servers, null when no default is configured).
• turbo test --filter=@posthog/api-client --filter=@posthog/ui and turbo typecheck for both packages - green.
• Not manually tested against a backend running the new endpoint yet.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 21ef863.}