Fix #232: add JavaTimeFeature.ALLOW_STRINGIFIED_DURATION_VALUES

[arimu1]

Fixes #232

Root cause

DurationDeserializer parses JSON Strings with Duration.parse, which only accepts ISO-8601 (PT…). Int-like Strings such as "3600" (OAuth expires_in) fail even when @JsonFormat(pattern = "SECONDS") is present, because that annotation only applies to JSON numbers.

Change

Add JavaTimeFeature.ALLOW_STRINGIFIED_DURATION_VALUES (default off, so existing behavior is unchanged). When enabled, integer and decimal numeric Strings are handled the same as JSON numbers:

• integers use @JsonFormat unit conversion / READ_DATE_TIMESTAMPS_AS_NANOSECONDS
• decimals use seconds with fractional nanos (same as JSON floats)

ISO-8601 duration Strings remain accepted either way.

Enable with:

JsonMapper.builder()
    .addModule(new JavaTimeModule()
        .enable(JavaTimeFeature.ALLOW_STRINGIFIED_DURATION_VALUES))
    .build();

Test plan

• DurationDeser232Test (10 tests) — default still fails "3600"; feature-on accepts "3600" / "-3600" / "60.5"; @JsonFormat(pattern="SECONDS") applies only when enabled
• Neighbors: DurationDeserTest, DurationDeser337Test, InstantDeser291Test
• Full mvn -pl datetime test on JDK 21 Temurin: 1143 tests, 0 fail, 1 skip

AI-assisted (Grok 4.6); human-reviewed.