Show environment pane close icon without hover

[captainsafia]

Description

• Keep the Environment Management pane header controls visible without requiring hover, so the close icon is always available like other always-visible pane headers.
• Extract the environment pane header construction into a helper and add a focused unit test for the always_show_icons behavior.

Linked Issue

No linked issue.

• 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

• cargo fmt -- --check
• cargo test -p warp test_environment_pane_header_always_shows_icons --lib
• cargo clippy --workspace --exclude warp_completer --all-targets --tests -- -D warnings
• cargo clippy -p warp_completer --all-targets --tests -- -D warnings
• I have manually tested my changes locally with ./script/run

Screenshots / Videos

Screenshot validation was delegated to a subagent. The sandbox cannot run live app screenshot capture because there is no DISPLAY/WAYLAND_DISPLAY and no available xvfb-run/Xvfb/weston, so the attached artifact is a labeled fallback visual, not a live Warp screenshot.

• Fallback visual artifact: environment_pane_close_icon_fallback.png (Oz conversation artifact 019e3cd6-7792-700b-90dc-e49240d9dce4)
• Original local artifact path from validation: /tmp/environment-pane-close-icon-screenshots/environment_pane_close_icon_fallback.png

Agent Mode

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

CHANGELOG-BUG-FIX: Fixed the Environment Management pane header so its close icon remains visible without hover.

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

Conversation: https://app.warp.dev/conversation/e8dc1eac-4971-4da6-8617-4f86ee8be27c
Run: https://oz.warp.dev/runs/019e3cba-7ffe-736a-8c67-2d41313809fa
This PR was generated with Oz.