fix: fall back for decimal SUM/AVG over sliding window frames (window audit)

[andygrove]

Which issue does this PR close?

Closes #4729

Rationale for this change

This PR is the result of auditing the native window function support added in #4209 (covering ranking, value-shift, and standard aggregate window functions). The audit walked the Scala serde (CometWindowExec), the native planner (create_window_expr / process_agg_func), and the DataFusion 54 window/aggregate accumulators, and verified behavior empirically through the differential SQL-file test harness.

The audit found one correctness divergence. SUM(<decimal>) over a sliding window frame (a frame whose lower bound is not UNBOUNDED PRECEDING) routes to DataFusion's built-in sum, whose accumulator wraps on overflow (add_wrapping) instead of returning Spark's NULL. On overflow the native result is a wrapped value that can fall outside the declared decimal precision, which even breaks Spark's result decoding (EXPRESSION_DECODING_FAILED / NUMERIC_VALUE_OUT_OF_RANGE). The divergence is decimal-specific: bigint sliding SUM overflow matches Spark (both wrap), and ever-expanding decimal SUM overflow already matches Spark (Comet's SumDecimal UDAF returns NULL).

What changes are included in this PR?

• Fall back to Spark for decimal SUM / AVG over a sliding window frame in CometWindowExec. Overflow cannot be detected at plan time, so the whole sliding decimal case falls back, mirroring the existing RANGE-frame DATE/DECIMAL fallbacks. Ever-expanding frames keep using Comet's overflow-aware SumDecimal / AvgDecimal UDAFs and continue to run natively.
• Add regression tests in windows/window_functions.sql (Section 8): ever-expanding decimal SUM overflow stays native and returns NULL, and sliding decimal SUM falls back.
• Record the findings in the window_funcs expression-audit notes.

The audit also surfaced a coverage gap that is not a correctness divergence and is tracked separately in #4731: AVG(<decimal>) over a window always falls back to Spark on Spark 4.x because CometWindowExec does not recognize the Cast(Divide(...)) average shape, leaving the AvgDecimal native window branch dead there. Results stay correct via fallback. The fallback guard added here also covers AVG so the overflow class of bug cannot reappear once that shape is recognized.

This work was scaffolded by the audit-comet-expression project skill.

How are these changes tested?

• New and existing cases in windows/window_functions.sql run through CometSqlFileTestSuite, which compares Comet against Spark.
• CometWindowExecSuite (49 tests) passes unchanged.

[comphead]

Thanks @andygrove