fix: ensure contiguous spans in span_range with exact=True and month frame (#1185)

[deepakganesh78]

Fixes #1185

Problem

span_range('month', start, end, exact=True) produces gaps (missing days) when the start day exceeds the number of days in a shorter month. For example, starting on Jan 31:

• Span 1: Jan 31 → Feb 27
• Span 2: Feb 28 → Mar 27
• Span 3: Mar 31 → … ← Mar 28-30 are missing

Root cause

range() clips the day when shifting into a shorter month (Jan 31 → Feb 28) and later restores it when months are long enough again (→ Mar 31). Each span(exact=True) computes a one-month window from its iteration point, so the clipped Feb 28 produces a ceiling of Mar 27, while the restored Mar 31 becomes the next floor — leaving a 3-day gap.

Fix

Track the previous span's ceiling; when the next computed floor would leave a gap, substitute prev_ceil + 1 µs as the floor instead, ensuring every moment between start and end is covered by exactly one span.

Validation

• Reproduction script confirms no gaps with the exact dates from the issue
• New regression test test_exact_month_no_gaps_when_day_clipped added
• Full test suite: 1901 passed, 2 skipped (pre-existing skips)

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.91%. Comparing base (2224255) to head (0a02d23).

Additional details and impacted files

@@             Coverage Diff             @@
##            master    #1332      +/-   ##
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- Coverage   100.00%   99.91%   -0.09%     
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  Files           10       10              
  Lines         2315     2321       +6     
  Branches       358      360       +2     
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+ Hits          2315     2319       +4     
- Misses           0        1       +1     
- Partials         0        1       +1     

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