fix: add horizontal scroll to rendered markdown code blocks

[yingxuel]

Description

Rendered markdown code blocks in the notebook file view were silently clipped at the pane edge with no affordance to reach overflowing content. Switching to Raw mode showed a horizontal scrollbar; Rendered mode had none.

Root cause: RenderableRunnableCommand drew paragraphs directly to the scene with no clip layer and no scroll state. Monospace code lines wider than the container extended past the right edge and were swallowed by the outer editor clip. (RenderableTable already has a full horizontal-scroll implementation; this brings code blocks to parity.)

Changes:

• crates/editor/src/render/element/paint.rs — adds draw_paragraph_scrolled, which temporarily increments RenderContext::nested_horizontal_scroll so that content_to_screen shifts text, selection highlights, cursors, and syntax-highlight decorations together under the scroll offset.
• crates/editor/src/render/element/runnable_command.rs — tracks scroll_left, natural_width, viewport_bounds, scrollbar, scrollbar_hovered, and scrollbar_drag; wraps code content in a BoundedByActiveLayerAnd clip layer; renders a horizontal scrollbar thumb (reusing the table scrollbar colours from the active theme); handles shift-scroll / horizontal trackpad and scrollbar drag in dispatch_event, mirroring RenderableTable.
• 10 unit tests covering: max-scroll arithmetic (fits / exact / overflow), ScrollData construction, delta gating (plain vertical → None, shift-scroll and horizontal trackpad → Some), line-multiplier scaling for imprecise scroll, and scroll_left clamping at both edges.

Linked Issue

Fixes #9089

• 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).

Screenshots / Videos

recording_9089.mov

Testing

• Added 10 unit tests in runnable_command.rs (see code_block_scroll_tests module); all pass with cargo nextest run -p warp_editor.
• Full warp_editor suite: 422/422 pass, zero regressions.
• Note on presubmit: 6 tests were already failing on master before this branch was created (verified by checking out master without these changes). These failures are pre-existing and unrelated to this PR.

Agent Mode

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

CHANGELOG-BUG-FIX: Rendered markdown code blocks now scroll horizontally when content overflows the pane width, matching Raw mode behaviour.

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@yingxuel

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Overview

This PR adds nested horizontal scrolling and scrollbar handling for rendered markdown runnable code blocks.

Concerns

• The new scroll offset only affects painting/event dispatch inside RenderableRunnableCommand; model-level hit testing for code blocks still uses unscrolled coordinates, so clicks, selection, cursor placement, and link hover can target the wrong text after horizontal scrolling.

Verdict

Found: 0 critical, 1 important, 0 suggestions

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[oz-for-oss]

⚠️ [IMPORTANT] Storing scroll_left only on the renderable means RenderState hit testing remains unaware of the horizontal offset; after scrolling, clicks/selection/link hover in the code block will resolve against unscrolled coordinates. Move the scroll state into the render model or otherwise feed it into code-block coordinate-to-location logic.