Support YYYY-MM date ranges

[ejsmith]

DateTimeRange.Parse("2025-01") and [2025-01 TO 2025-01] currently fall back to the unrestricted Empty range. Add native YYYY-MM parsing so they resolve to January 1 through January 31, using the supplied offset and existing boundary-inclusivity rules. This enables exceptionless/Exceptionless#2617 to remove its API-side month expansion after adopting the updated package.

Reuse the month boundary handling in DateMath for standalone ranges and range components. This intentionally corrects DateMath.Parse("2025-01", baseTime, isUpperLimit: true) from January 1's end to January 31's end. Named timezones resolve their offset at each boundary, including DST changes; existing year, day, and timestamp formats remain covered by regression tests.

Validation:

• Build passes for net8.0 and net10.0 with zero warnings.
• All 802 tests pass, including month lengths, leap years, offsets, DST, all bracket combinations, comparisons, mixed precision, invalid input, date limits, and dash-separated years. Local tests use temporary TZDIR aliases for existing US/* timezone tests; no system timezone files were changed.
• Ran dotnet format and git diff --check.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-01T16:07:50.121050Z	3261845	PR opened

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