Refine markdown path detection in CodeView

[qkal]

Summary

• Deduplicate CodeView markdown path detection into shared helpers so markdown-specific UI decisions use the same precedence.
• Apply that shared detection to both the markdown preview toggle and the left-padding behavior for markdown tabs.
• Add focused regression tests covering path precedence: live local path first, then tab path, then source path.

Verification

• cargo fmt --all
• cargo test -p warp-app markdown_path_detection --lib (3 passed; 0 failed; 3992 filtered out)
• git diff --check

[Copilot]

Pull request overview

This PR refactors how CodeView decides whether the active view should be treated as “Markdown”, centralizing path-precedence logic so both the Markdown preview toggle and Markdown-specific layout behavior follow the same rules.

Changes:

• Introduces shared helpers to pick a preferred path (local path → tab path → source path) and detect Markdown from it.
• Reuses the shared Markdown detection for both the Markdown mode segmented control and the render-time padding behavior.
• Adds focused unit tests covering the intended precedence order for Markdown detection.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.