Handle X11 attention send failures

[warp-agent-staging]

Prevent request_user_attention from panicking when X11 rejects the WM-hints request because the connection has already closed. A successfully sent request still discards its cookie without waiting for a reply.

Warp pins this revision in warpdotdev/warp-for-benchmarks#57.

• Tested on all platforms changed — Linux/X11 tests and clippy passed.
• Added an entry to the changelog module if knowledge of this change could be valuable to users — omitted for this downstream maintenance fix.
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior — no public API or documented behavior changed.
• Created or updated an example program if it would help users understand this functionality — no example is needed for connection-loss handling.
• Updated feature matrix, if new features were added or implemented — no feature changed.

Validation:

• cargo test --no-default-features --features x11
• cargo clippy --all-targets --no-default-features --features x11 -- -Dwarnings

The dead-connection branch was not manually reproduced because it requires invalidating the X11 transport between window creation and the attention request.

[warp-agent-staging]

This PR was generated with Warp.

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