Build helper tests

Live under crates/ethercat-esi-build/tests/.

Test Case: Builder writes a parseable Rust file to OUT_DIR TEST_0440
status: open
verifies: REQ_0540, REQ_0541

Test crate driven by a fixture ESI set runs Builder::new().glob(...).backend(default).out_file( "devices.rs").build() in a tempfile-backed OUT_DIR. Asserts the file exists, is non-empty, and parses with syn::parse_file (catches malformed token streams).
Test Case: cargo rerun-if-changed emitted per ESI input TEST_0441
status: open
verifies: REQ_0542

Capture Builder::build()’s stdout output (the build helper prints to stdout per cargo conventions). For an N-file glob, assert exactly N + 1 cargo:rerun-if-changed= lines are present (one per ESI file + one for the build script itself).
Test Case: Output passes prettyplease formatting TEST_0442
status: open
verifies: REQ_0543

The generated devices.rs file is line-wrapped (no line exceeds 100 chars without justification), and re-running prettyplease::unparse on the file produces a byte-identical output (idempotent formatter pass).