Group plaintext lines in comment editors

[alokedesai]

Summary

• Add an opt-in group_plain_text_lines render layout option for rich-text editors.
• Enable the option for code comment editors so adjacent plaintext lines typed with Enter are rendered as one compact TextBlock instead of separate paragraph blocks.
• Coalesce adjacent plaintext render items after incremental edits so the typed Enter path matches reset-from-markdown behavior.
• Keep rich block rendering and global rich-text spacing constants unchanged.
• Add low-level editor and comment-editor regression tests for grouped plaintext lines.

Validation

• cargo test --manifest-path /workspace/warp/Cargo.toml -p warp_editor test_layout_text_block_groups_plain_text_lines_when_enabled
• cargo test --manifest-path /workspace/warp/Cargo.toml -p warp code::editor::comment_editor::tests
• cargo build --manifest-path /workspace/warp/Cargo.toml -p warp --bin warp
• cargo build --manifest-path /workspace/warp/Cargo.toml -p warp --bin integration
• Computer-use demo in the real inline code-review comment editor: typed two plaintext lines separated by Enter; observed compact adjacent lines with ~15–16px baseline distance and ~1–3px visible gap.

Co-Authored-By: Oz oz-agent@warp.dev

Conversation: https://staging.warp.dev/conversation/2c4d5e82-c039-41cf-beed-436f3e14aaad
Run: https://oz.staging.warp.dev/runs/019e42e0-1eb6-71dd-a917-5dd858192cc8
This PR was generated with Oz.