Add ensure! macro for BevyError

[0xEgao]

Objective

Bevy provides bevy_error! and bail! for error handling, but lacks a concise way to return an error when a condition is false.

Fixes #24645.

Solution

• Add an ensure! macro that delegates to bail! when its condition is false.
• Support the existing error-message formatting and optional Severity syntax.
• Add API documentation, regression test and usage examples.

Testing

• RUST_BACKTRACE=1 cargo test -p bevy_ecs --lib
• cargo test -p bevy_ecs --doc ensure
• cargo clippy -p bevy_ecs --lib -- -D warnings
• cargo fmt --all -- --check

Showcase

ensure!(score >= 0, "score must not be negative: {}", score);

ensure!(
    score <= 100,
    Severity::Warning,
    "score is too high: {}",
    score
);

[alice-i-cecile]

Needs a cross-link from bail! for discoverability :)

[0xEgao]

Needs a cross-link from bail! for discoverability :)

Done :)