ICU-23337 Fix undefined behavior in list_format_fuzzer: validate enum values

[OwenSanzas]

Summary

The list_format_fuzzer harness reads UListFormatterType and UListFormatterWidth directly from fuzz data via memcpy into enum variables without validation. Loading arbitrary bit patterns into C++ enum types is undefined behavior.

Confirmed with UndefinedBehaviorSanitizer:

runtime error: load of value 2125315823, which is not a valid value
for type 'UListFormatterType' (aka 'enum UListFormatterType')
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior list_format_fuzzer.cpp:52:52

The harness already validates these values for a third createInstance call (lines 58-61), but passes the raw unvalidated values in the second call (line 52).

Changes

1. Read fuzz data into int32_t instead of directly into enum types (eliminates UB at the memcpy point)
2. Validate values with abs + modulo before casting to the enum type
3. Remove the redundant third createInstance call — it was only needed because the second call used unvalidated values; now the second call uses validated values directly

Experimental Verification (60-second fuzzing, AddressSanitizer)

Metric	Original	Fixed
Edge cov	1658	1651
Features	4218	3873
Corpus	314	261
Exec/s	9832	9913

Coverage is equivalent — the fix eliminates the UB without affecting fuzzing effectiveness.

UBSan Evidence

• Original: crashes with crafted input containing type=0x7EADBEEF, width=0x7EADBEEF
• Fixed: passes cleanly with the same input