fix(desktop): stop the create-agent provider config probe from erasing keystrokes

[tlongwell-block]

What

Fixes the create-agent dialog's "Run on" provider config fields eating keystrokes — reported by Tyler in buzz-remote-agents (channel 29414326, thread db76677a): the Kubernetes Kubeconfig context field would not accept typing.

Why it happened (the Typewriter Eraser, shipped in #4289)

WhereToRunSection's probe useEffect depended on the whole draft:

1. every keystroke changed the draft → effect re-fired → provider binary re-probed;
2. each probe result is a fresh object written into the draft → the effect re-triggered itself, respawning the provider binary in a loop for as long as the dialog sat on a provider;
3. every probe resolution reset providerConfig to schema defaults — erasing whatever was typed. A field with no schema default (context) snapped back to empty, i.e. "won't let me type". Unrelated to how many kubeconfig contexts you have.

Fix

• Probe once per provider selection, keyed on the provider's stable binaryPath — not the draft, not the provider object (a useBackendProvidersQuery refresh must not reprobe an unchanged selection).
• Latest-state resolution via React.useEffectEvent + a new pure applyProbeResult helper: schema defaults merge beneath the current providerConfig, so a probe landing after the user typed can never clobber in-flight input (per Wren's pre-patch red-team: changing deps alone leaves a stale closure).

Existing cancelled cleanup keeps provider-switch/unmount safe; selection reset (emptyWhereToRunDraft) and the fail-closed probe-error path are unchanged.

Tests

• Unit (whereToRunIntent.test.mjs): applyProbeResult merge semantics — defaults under typed values, user-cleared fields stay cleared, schema-less results, unrelated fields preserved.
• E2E (new where-to-run-config.spec.ts, added to the smoke project, red-first verified: all 3 fail against the unfixed component):
  • typing into a defaultless provider field sticks, and probe_backend_provider fires exactly once per selection;
  • the config form is gated on probe resolution (slow probe: no half-rendered form, defaults prefill once);
  • provider → local → provider re-probes and resets cleanly.
• Mock bridge gains backendProviders / backendProviderProbeResult / backendProviderProbeDelayMs seams (defaults preserve prior behavior).

Verification at 8eb7680

• pnpm check + tsc clean, pnpm test 3926/3926;
• new spec 3/3 green (and 3/3 red on the unfixed component);
• pre-push lefthook: desktop-test, desktop-check, desktop-tauri-checks, rust-tests, mobile-test all green.

[Chessing234]

the useEffectEvent + path-keyed probe fix reads right — depending on draft was a nasty eraser loop. glad this landed in the create-agent dialog.