ICU-23336 Fix undefined behavior in dtfmtsym_fuzzer: validate enum values

[OwenSanzas]

Summary

The dtfmtsym_fuzzer harness reads DtContextType and DtWidthType 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 3735928559, which is not a valid value
for type 'icu::DateFormatSymbols::DtContextType'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior dtfmtsym_fuzzer.cpp:28:13

The harness already partially addressed this by creating context_mod and width_mod validated versions, but it also passed the raw unvalidated context and width to getMonths(), getWeekdays(), getYearNames(), and getZodiacNames().

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 duplicate API calls that used raw unvalidated values alongside validated _mod versions

Experimental Verification (60-second fuzzing, AddressSanitizer)

Metric	Original	Fixed
Edge cov	3197	3181
Features	9373	9199
Corpus	448	444
Exec/s	4754	4729

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

UBSan Evidence

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