Expand hidden diff sections fully on double-click

[bradleyjames]

Description

Single-clicking a hidden-section gutter button in the diff editor reveals one chunk at a time. This PR adds double-click as a shortcut to fully expand the entire hidden section in one action, instead of needing to click 3–5 times for a large hidden range.

To avoid the chunk briefly flashing before the full expand on a real double-click, the single-click action is deferred for the system's configured double-click interval (NSEvent.doubleClickInterval on macOS, GetDoubleClickTime on Windows, 500ms fallback elsewhere) and canceled if a second click arrives within that window. Result: a single click feels normal (after a barely-perceptible debounce) and a double click goes straight to full expansion with no transient chunk visible.

The disambiguation lives behind a new platform primitive — warpui_core::DeferredCall — a cancellable, RAII single-pending deferred callback for views. The diff editor consumes it; any future single-vs-double-click feature can reuse it instead of rebuilding the timer + abort dance.

Linked Issue

Closes #11622

• The linked issue is labeled ready-to-spec or ready-to-implement.
• Where appropriate, screenshots or a short video of the implementation are included below (especially for user-visible or UI changes).

Testing

• I have manually tested my changes locally with ./script/run

Automated coverage:

• Unit tests for the new DeferredCall primitive (crates/warpui_core/src/core/view/deferred_tests.rs): four cases covering schedule firing, cancel aborting, Drop auto-canceling, and reschedule superseding a prior pending call.
• Integration test test_diff_editor_double_click_fully_expands_hidden_section in crates/integration/src/test/code_review.rs: boots a real Warp instance, sets up a git repo with a 400-line file producing three hidden ranges, opens the code review panel, and verifies that dispatching the HiddenSectionExpansion action with click_count: 2 fully removes the first hidden section.
• All six pre-existing code-review integration tests still pass alongside the new one — verified locally with cargo run -p integration --bin integration -- <test_name>.

Manual coverage:

• Single-click on large hidden section → chunked expansion after ~500ms debounce, no flash.
• Double-click on the same section → full expansion immediately, no chunk visible mid-flight.
• Two slow single-clicks (>OS double-click window apart) → each chunks independently.
• Adjacent gutter interactions (diff-hunk sliver toggle, add-context, revert) unchanged.

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-IMPROVEMENT: Double-click a collapsed hidden section in the diff editor to fully expand it in one action.

[oz-for-oss]

@bradleyjames

This PR is not linked to an issue that is marked with ready-to-implement.

Issue-state enforcement details:

• Associated same-repo issues checked: none

• Required readiness label: ready-to-implement

To continue, link this PR to a same-repo issue such as Closes #123 in the PR description, and make sure that issue has ready-to-implement.

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[oz-for-oss]

@bradleyjames

This PR is not linked to an issue that is marked with ready-to-implement.

Issue-state enforcement details:

• Associated same-repo issues checked: none

• Required readiness label: ready-to-implement

To continue, link this PR to a same-repo issue such as Closes #123 in the PR description, and make sure that issue has ready-to-implement.

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