Time Unit Unification for bin/stats

[ahkcs]

Description

Implements unified time unit support across PPL commands with case-sensitive handling.

Key Changes:

• Fixed case sensitivity for bin command: Uppercase M = month, lowercase m = minute
• Added subsecond test cases to bin command: us, cs, ds
• Documented limitations: stats command cannot support subsecond units (OpenSearch calendar intervals)

Related Issues

Resolves #4397

[dai-chen]

Shall we have single place for span parsing logic (currently in AstExpressionBuilder.visitSpan?) instead of trying to parse span string everywhere? Any difference between span in bin options and span elsewhere?

[ahkcs]

Shall we have single place for span parsing logic (currently in AstExpressionBuilder.visitSpan?) instead of trying to parse span string everywhere? Any difference between span in bin options and span elsewhere?

Thanks @dai-chen for the suggestion, however bin span is indeed different with the span elsewhere
For Span Types:

• stats/timechart: Numeric + Time only
• bin: Numeric + Time + Logarithmic (log2, log10, 2.5log3)

I think it's still necessary to separate the implementation of bin span and other span