Add WorkerSend background spawn lint

[miguelraz]

Summary

• add a Dylint for GPUI background tasks containing data that does not implement WorkerSend
• emulate auto-trait behavior for closure captures, coroutine state, nested futures, and opaque futures
• ignore call sites that are provably excluded from wasm and cover positive and negative cases with UI tests

Tests

• cargo fmt --manifest-path tooling/lints/Cargo.toml --all -- --check
• cargo +nightly-2026-03-21 test --manifest-path tooling/lints/Cargo.toml

Release Notes:

• N/A