ci: enforce warnings as errors in test entrypoints

[clydegerber]

Summary

• Adds RUSTFLAGS="-D warnings" to run_cargo_test() and run_cargo_doc() in build/entrypoint.sh

All compiler warnings have been addressed in the preceding PRs (#349, #350, #356). This commit promotes warnings to errors in CI to prevent regressions.

Test plan

• CI passes with -D warnings enabled
• No new compiler warnings are introduced

This commit was created by an automated coding assistant, with human supervision.

[clydegerber]

Can I get a review of this?

[clydegerber]

@filmil — friendly bump. Three open PRs awaiting review:

• ci: enforce warnings as errors in test entrypoints #357 (32 days): -D warnings in CI. All compiler warnings have been addressed in fix(sys): reinstate deref_nullptr lint #349, fix(sys): address compiler warnings in bindgen configuration #350, fix: address compiler warnings across crates #356 (all merged).
• chore(sys): remove unused lint warning suppressions #361 (13 days): removes unused dead_code, non_upper_case_globals, unused_imports, rustdoc::bare_urls suppressions from rust_icu_sys. Verified locally with cargo test — no warnings emerge.
• refactor: remove redundant ICU version-detection mechanism #364 (13 days): removes redundant ICU version gates (every supported ICU version is ≥ 74, making _64_plus/_67_plus/_68_plus always true; discussion in the PR description). Tested all CI matrix configurations locally.

All three are mergeable with no conflicts. Let me know if any of them need rework, or if you'd prefer a different merge order.

[filmil]

Can I get a review of this?

Weird, I do not rememebr seeing this nudge. In any case thank you for the nudge.

[filmil]

Looking at this now, I rememebr reviewing this, but not approving it. :/