🧹 Refactor tests to remove unsafe unwrap() in uf2.rs

[undivisible]

🎯 What: Removed unsafe .unwrap() usages when reading u32 values from byte slices in the test assertions of in-cli/src/native_emit/uf2.rs.
💡 Why: To improve code health and maintainability by introducing a robust error handling approach with a read_u32 helper function that safely handles the conversion.
✅ Verification: Verified by checking formatting with cargo fmt, running lints with cargo clippy, running local module tests with cargo test native_emit::uf2, and running the full test suite with cargo test in the in-cli crate. All steps completed successfully with zero failures.
✨ Result: The tests no longer panic with unwrap() on slice length mismatches and safely propagate string errors.

Note: Automated code review hallucinated that returning Result<(), String> from the tests when using ? on encode_uf2(...)? would fail compilation. However, encode_uf2 itself already returns Result<Vec<u8>, String>, matching the test return types seamlessly. Local compilation and test execution confirm this change is perfectly valid and completely safe.

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PR created automatically by Jules for task 1027258785939123781 started by @undivisible

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Note

Low Risk
Test-only changes in uf2.rs; production UF2 encoding behavior is untouched.

Overview
Refactors UF2 encoder unit tests in uf2.rs to avoid panicking on slice conversions and encoding failures.

A test-only read_u32 helper replaces repeated try_into().unwrap() + from_le_bytes when asserting header fields. encodes_single_block_with_family and splits_large_payload now return Result<(), String> and use ? on encode_uf2 and read_u32, so assertion/setup errors surface as test failures instead of panics. rejects_empty is unchanged and still uses expect_err.

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