fix(cargo): preserve test compile diagnostics

[youbamj]

Summary

• preserve rustc compile diagnostics when cargo test fails before any test suites run
• reuse the existing build diagnostic formatter for compile-error-only cargo test output and relabel it from cargo build: to cargo test:
• add a regression test and update CHANGELOG.md for the cargo test compile-diagnostics fix

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test
• Unit test added in src/cargo_cmd.rs for compile-failing cargo test output
• Manual testing: created a temporary compile-failing test and ran ./target/debug/rtk cargo test --test repro_compile_fail
• Manual testing: verified filtered output preserved error[E0425] and tests/repro_compile_fail.rs:3:13
• Manual testing: removed the temporary repro file after verification

Closes #740

[aeppling]

Hello,

Thanks for this clean contribution !

For me this is explicit and straight-forward, we can merge this