feat: Add cum_mean Expression and Series method

[alexander-beedie]

Also closes #26065.

Been working on this one on & off for a few weeks; ready for review 🤔

Since we need to track state for two variables, added a CumAggState enum...

pub enum CumAggState {
    Single(AnyValue<'static>),
    Mean(CumMeanState),
}

...with all the existing cumulative exprs using the single-value option, and mean using the new CumMeanState, which tracks the sum and count vars (for float and -if indicated- decimal). Can extend in the future if any other aggregate functions need additional state tracking.

The cumulative mean calculation itself is relatively unexciting (though there is a dedicated Decimal path, patterned after the same in the other cumulative funcs). It got much better this morning when I found the existing KahanSum struct and was able to (almost) just drop it right in, making the overall implementation feel much improved 👍

We had (relatively) little dedicated cumulative expression coverage, so I added more in a new test_cumulative.py file, covering additional validations such as streaming vs in-memory, optimised/unoptimised, nan poisoning, inf behaviour, and the full range of int/float/temporal dtypes (slightly over half of this PR is just new test coverage).

---

🤖 AI: Claude Code (Opus 4.6) helped identify test coverage gaps and provided the initial unit test skeleton, which I then industrialised and made more idiomatic.

[codecov]

Codecov Report

❌ Patch coverage is 89.41799% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.39%. Comparing base (2892ced) to head (f8abe95).

Files with missing lines	Patch %	Lines
crates/polars-ops/src/series/ops/cum_agg.rs	90.09%	11 Missing ⚠️
crates/polars-stream/src/nodes/cum_agg.rs	89.74%	4 Missing ⚠️
crates/polars-plan/src/dsl/function_expr/mod.rs	0.00%	2 Missing ⚠️
...s/polars-plan/src/plans/aexpr/function_expr/cum.rs	90.00%	1 Missing ⚠️
...s/polars-plan/src/plans/aexpr/function_expr/mod.rs	66.66%	1 Missing ⚠️
.../polars-python/src/lazyframe/visitor/expr_nodes.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26569      +/-   ##
==========================================
+ Coverage   81.38%   81.39%   +0.01%     
==========================================
  Files        1792     1792              
  Lines      244414   244583     +169     
  Branches     3078     3078              
==========================================
+ Hits       198913   199084     +171     
+ Misses      44715    44713       -2     
  Partials      786      786              

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