⚡ perf: replace expect(&format!(...)) with unwrap_or_else to avoid unnecessary allocations

[filmil]

💡 What: Replaced the use of .expect(&format!(...)) with .unwrap_or_else(|e| panic!(...)) in the test suites of rust_icu_ecma402/src/numberformat.rs and rust_icu_ecma402/src/datetimeformat.rs.
🎯 Why: The format! macro allocates a new String every time it's called. When used inside expect(), this allocation happens on every execution, including the success path. Using unwrap_or_else ensures that formatting and allocation only occur if a panic is actually triggered.
📊 Measured Improvement: A meaningful performance improvement could not be demonstrated via benchmarks due to environment limitations (missing libicu-dev and inability to download crates offline). However, this is a standard Rust performance optimization that eliminates unnecessary allocations in hot paths or frequently run tests.

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

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PR created automatically by Jules for task 12913409335323361221 started by @filmil

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