Fix master CI: restore child_view_ids on StoredView

[warp-dev-github-integration]

Description

master CI went red after #12633 merged. The merge produced a semantic
conflict: #12633 introduced the StoredView enum (wrapping AnyView /
AnyTuiView) as the type stored in each window's views map, but a
concurrently-merged change — AppContext::collect_transferable_subtree —
still calls child_view_ids on a window's stored view. StoredView did not
delegate that neutral hook, so warpui_core failed to compile:

error[E0599]: no method named `child_view_ids` found for reference
  `&core::window::StoredView` in the current scope
  --> crates/warpui_core/src/core/app.rs:3306:39

This broke every CI job in the suite (clippy on all platforms, all test
jobs, and the release-flag compilation jobs).

Changes

• Add StoredView::child_view_ids, delegating to the GUI view's
  implementation and returning an empty vec for TUI views (matching the
  existing TUI no-op delegations like accessibility_data).
• Fix missing TuiBuffer / TuiConstraint / TuiRect / TuiSize imports in
  tui_view_tests.rs, which broke the tui-feature lib+test build. (No CI
  job builds with --features tui, but this restores that configuration.)

Testing

• cargo check -p warpui_core — clean (default and --features tui).
• cargo clippy -p warpui_core --all-targets -- -D warnings — clean (default
  and --features tui).
• cargo fmt -p warpui_core -- --check — clean.
• cargo nextest run -p warpui_core transfer_view — 14/14 pass (exercises the
  child_view_ids path through StoredView).

Agent Mode

• Warp Agent Mode

CHANGELOG-NONE

Conversation: https://staging.warp.dev/conversation/d1c84ef4-952f-481e-9b2a-2ef61bbf9bb4
Run: https://oz.staging.warp.dev/runs/019ee255-ee44-75a1-9484-56c988702c4a

This PR was generated with Oz.