fix(web): preserve HITL input name on prefill selection

[luochen211]

Summary

Fixes #37787.

• Preserve the Human Input Save response as value when the pre-populate variable picker commits a selector update.
• Update paragraph default-value changes to derive from the latest temporary payload instead of a previously rendered payload snapshot.
• Add regression coverage for a stale pre-populate selector callback so the output variable name remains available after selecting an upstream variable.

Verification

• pnpm -C web test app/components/base/prompt-editor/plugins/hitl-input-block/__tests__/input-field.spec.tsx
• pnpm -C web type-check
• pnpm lint --quiet web/app/components/base/prompt-editor/plugins/hitl-input-block/input-field.tsx web/app/components/base/prompt-editor/plugins/hitl-input-block/__tests__/input-field.spec.tsx

Risk

• Scope: limited to the HITL input field editor state update path.
• Regression risk: low; select/file input paths are unchanged, and the existing input-field test suite still passes.