Fix updater runtime error state

[bearyung]

Summary

• remove a managed runtime provider's stale error key after a successful install
• treat only non-empty runtime error messages as failed checks in updater state and the Updates dialog
• add regression coverage for the current-state transition and two-provider error preservation

Root cause

Successful runtime installation assigned undefined to the provider entry in runtimeErrors. The Updates dialog checked only the number of object keys, so the stale key produced a false "Some update checks failed" state while rendering no provider-specific warning.

User impact

After installing managed Codex or Claude runtime updates, the dialog now immediately reports the correct current state. A genuine error for the other provider remains visible.

Validation

• npx vitest run src/lib/state/updater.test.ts
• npm run check
• npm test
• npm run build
• git diff --check