feat: add Character and Year converters (#1017)

[BigDataDZ]

Purpose of the pull request

Implements two commonly used JDK 8 compatible converters requested by the community task #1017:
java.lang.Character and java.time.Year. Proposal comments:
#1017 (comment) and the amendment below it
(UUID / YearMonth / Instant were proposed by other volunteers in parallel and are left to
them).

Related: #1017

What's changed?

• CharacterStringConverter (org.apache.fesod.sheet.converters.charconverter): bidirectional
  Character <-> STRING. Reading an empty string yields null; strings longer than one character
  throw (wrapped into ExcelDataConvertException by the framework) instead of being silently
  truncated.
• YearStringConverter (org.apache.fesod.sheet.converters.year): bidirectional Year <->
  STRING, default format uuuu, honors @DateTimeFormat custom patterns and
  GlobalConfiguration.locale.

Both follow the existing *StringConverter pattern (e.g. LocalTimeStringConverter) and are
registered in DefaultConverterLoader under putAllConverter, putWriteConverter and
putWriteStringConverter, so they work on read, xlsx write and CSV write alike.

Each converter ships with JUnit 5 unit tests (11 tests total: type keys, round-trips, default and
custom formats, and error cases).

Checklist

• I have read the Contributor Guide.
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

[BigDataDZ]

Done in 254a11e — renamed to DateUtils.defaultYearFormat, following the sibling fields
defaultDateFormat / defaultLocalDateFormat, and parseYear / format(Year, ...) now fall
back to it when no explicit format is given, so it works as a global default just like the other
two. Tests re-run green (11/11 targeted, 927/927 full suite).

[bengbengbalabalabeng]

It is recommended to add tests for the following scenarios to cover the complete behavior of YearStringConverter / DateUtils:

• DateUtils#parseYear() / format(Year)
• Locale's impact on formatting（format = "G y"，Locale.CHINA -> "公元 2026"，Locale.US -> "AD 2026"）

[BigDataDZ]

Added in faf2207, covering both layers:

1. DateUtilsTest — direct coverage for DateUtils#parseYear / #format(Year, ...):
   default yyyy parse and format (with null and explicit locales), plus a custom pattern
   (uu).
2. Locale coverage — format(Year.of(2026), "G y", Locale.CHINA) → 公元 2026 and
   Locale.US → AD 2026, mirrored through the converter in
   YearStringConverterTest#convertToExcelDataRespectsLocaleForEraPatterns (a
   GlobalConfiguration with Locale.CHINA / Locale.US and a @DateTimeFormat pattern of
   G y).

Also adjusted the existing custom-format test: the previous MM/yyyy pattern is invalid for a
bare Year (no MonthOfYear field — UnsupportedTemporalTypeException), replaced with
'Y'yyyy → Y2026.

Targeted tests 79/79, full fesod-sheet suite 933/933 green.