fix: defer code review editor construction for collapsed files (APP-4518)

[warp-dev-github-integration]

Description

Fix a memory spike in code review diff rendering (Sentry issue #7259255054) where CodeReviewView::build_view_state_for_file_diffs eagerly constructs editor state (LocalCodeEditorView, CodeEditorView, CommentEditor, GlobalBufferModel) for every file diff — including collapsed files that are never rendered.

Root cause: Editor and global-buffer construction happened unconditionally for all non-binary files. Only afterward was the expansion state checked. For large PRs with many files, the majority are collapsed (binary, autogenerated, or large), yet each still materialized ~50-100MB of styled buffer data, pushing total in-use heap to ~9 GiB.

Fix: Check the expansion state before creating the editor. Collapsed files now store only the lightweight content_at_head string in a new deferred_content_at_head field on FileState. The full editor is created lazily via ensure_editor_for_file() when the user expands the file via ToggleFileExpanded or FileSelected.

Expected impact from heap profile:

• CodeEditorView constructors: ~4.37 GiB → 0 for collapsed files
• CommentEditor::new: ~3.25 GiB → 0 for collapsed files
• GlobalBufferModel allocation: significant reduction for collapsed files

Note: This supersedes the approach in PRs #11275 and #11250, which attempted the same fix but remain unmerged.

Linked Issue

• APP-4518

• Sentry #7259255054

• 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

• Verified the change compiles cleanly with cargo clippy -p warp --all-targets -- -D warnings ✅

• Ran cargo fmt with no formatting issues ✅

• Existing unit tests in code_review_view_tests.rs pass (updated test to include new deferred_content_at_head field)

• The change is behavioral: collapsed files now start without editors, and editors are created on-demand when expanded. All existing expansion/selection/comment code paths that reference editor_state already handle None gracefully.

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

Agent Mode

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

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