test: [6598098782] comprehensive tests for indefRows interval fill

Add test_fun_ts_interval_fill.py covering 26 test cases for all
timeseries functions with INTERVAL + FILL:
- Basic fill modes (null/none/value/prev/next/linear) for each function
- Unsupported fill mode rejection for indefRows functions
- Dense windows, null input, single-row windows, consecutive windows
- Gap patterns: leading, trailing, multiple consecutive gaps
- Interval size variations (10s, 500ms)
- Super-table partitioned queries
- Negative values and INT32 boundary values
- Empty result range
- Large dataset (85 rows, 20 windows)
- Cross-block boundary (997 rows/window, maxrows=200)
- Nested queries, LIMIT/OFFSET
- Regular agg fill unchanged verification

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

Author: Simon9997