feat: implement stddev with ddof parameter support

[venkateshdb]

Changes Made

• Added ddof param to stddev in DataFrame, GroupedDataFrame, and Expression (default 0 = population; 1 = sample).
• Introduced StddevParams and updated AggExpr::Stddev to carry ddof through DSL, IR, logical, and physical plans.
• Implemented population vs sample stddev formulas in planners, updated Python bindings to stddev(ddof) and SQL to support stddev (ddof=0) and stddev_samp (ddof=1).
• Allowed Null input type for stddev supertype.

Related Issues

Checklist

• [ ] Documented in API Docs (if applicable)
• [ ] Documented in User Guide (if applicable)
• [ ] If adding a new documentation page, doc is added to docs/mkdocs.yml navigation
• [ ] Documentation builds and is formatted properly (tag @/ccmao1130 for docs review)

[srilman]

@venkateshdb I'm going to make this PR a draft since CI is failing. Can you open up the PR for review once you're ready?

[codecov[bot]]

Codecov Report

:x: Patch coverage is 76.57658% with 26 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 74.26%. Comparing base (1f59628) to head (98bc7b9).

Files with missing lines	Patch %	Lines
src/daft-logical-plan/src/ops/project.rs	0.00%	9 Missing :warning:
src/daft-core/src/array/ops/stddev.rs	50.00%	4 Missing :warning:
daft/dataframe/dataframe.py	50.00%	3 Missing :warning:
src/daft-core/src/python/series.rs	0.00%	3 Missing :warning:
src/daft-core/src/series/ops/agg.rs	57.14%	3 Missing :warning:
src/daft-core/src/utils/stats.rs	85.71%	2 Missing :warning:
src/daft-dsl/src/expr/mod.rs	85.71%	2 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4949   +/-   ##
=======================================
  Coverage   74.26%   74.26%           
=======================================
  Files         956      956           
  Lines      123101   123184   +83     
=======================================
+ Hits        91424    91487   +63     
- Misses      31677    31697   +20     

Files with missing lines	Coverage Δ
daft/expressions/expressions.py	96.55% <100.00%> (ø)
src/daft-core/src/datatypes/agg_ops.rs	62.06% <100.00%> (+0.66%)	:arrow_up:
src/daft-dsl/src/python.rs	90.46% <100.00%> (ø)
...ft-physical-plan/src/physical_planner/translate.rs	85.92% <100.00%> (+0.16%)	:arrow_up:
src/daft-recordbatch/src/lib.rs	78.61% <100.00%> (ø)
src/daft-sql/src/modules/aggs.rs	72.00% <100.00%> (+2.70%)	:arrow_up:
src/daft-core/src/utils/stats.rs	94.59% <85.71%> (-3.80%)	:arrow_down:
src/daft-dsl/src/expr/mod.rs	79.55% <85.71%> (-0.14%)	:arrow_down:
daft/dataframe/dataframe.py	78.72% <50.00%> (-0.14%)	:arrow_down:
src/daft-core/src/python/series.rs	84.69% <0.00%> (-0.84%)	:arrow_down:
... and 3 more

... and 8 files with indirect coverage changes

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[venkateshdb]

Hi @srilman, Can you review this PR and can you let me know what changes are needed.

[srilman]

@venkateshdb Do you mind responding to Greptiles comments first, whether or not they are reasonable or not? If they are, can you address them?

[malcolmgreaves]

👋 Hi @venkateshdb ! Thanks for making this contribution. Just checking in here -- how is it going addressing the review feedback?

[madvart]

@venkateshdb Thanks again for contributing. Would you be able to address the reptile feedback, so we can then review this?

[madvart]

Hi @venkateshdb - Checking to see if you are still planning to work on this?

[venkateshdb]

@madvart, Yes