[PR] feat: support dayname and monthname expressions [datafusion-comet]

["andygrove (via GitHub)" <gi...@apache.org>]

andygrove opened a new pull request, #4544:
URL: https://github.com/apache/datafusion-comet/pull/4544

   ## Which issue does this PR close?
   
   Part of #4418.
   
   ## Rationale for this change
   
   `dayname` and `monthname` (Spark 4.0+) had no Comet support and forced a fallback to Spark. While auditing the datetime functions for this gap, it also became clear that several functions marked as unsupported in the coverage document already execute in Comet: Spark rewrites them to expressions Comet already supports before the plan reaches Comet, so the coverage document understated what works.
   
   This PR closes the `dayname` / `monthname` gap and corrects the documentation for the rewrite-backed functions, so users are not misled into thinking common functions like `to_timestamp` or `current_timestamp` are unaccelerated.
   
   ## What changes are included in this PR?
   
   This work was scaffolded with the `implement-comet-expression` skill.
   
   - **`dayname` / `monthname`**: these are Spark 4.0+ only, so they cannot be referenced from shared serde code. They are wired in the spark-4.0 `CometExprShim` and routed through the Arrow-direct codegen dispatcher (`CometScalaUDF.emitJvmCodegenDispatch`), which runs Spark's own `doGenCode` for exact parity (including locale handling). When the dispatcher is disabled they fall back cleanly.
   - **Documentation**: `spark_expressions_support.md` is updated to mark the following as supported, each with a note explaining the path:
     - `to_date` / `to_timestamp` / `to_timestamp_ntz` / `to_timestamp_ltz`: rewrite to `Cast` (no format) or `GetTimestamp` (with format).
     - `make_timestamp_ntz` / `make_timestamp_ltz` (6-argument form): rewrite to `MakeTimestamp`. The 2-argument `(date, time)` form needs the Spark 4.1 TIME type and still falls back.
     - `current_date` / `current_timestamp` / `now` / `curdate`: constant-folded to literals by Spark's `ComputeCurrentTime` rule before Comet sees the plan.
   
   ## How are these changes tested?
   
   New tests in `CometExpressionSuite` use `checkSparkAnswerAndOperator` to assert each function executes fully in Comet (no fallback) and returns Spark-identical results, plus a constant-folding assertion for `current_date` / `current_timestamp` / `now`. The `dayname` / `monthname` test is gated to Spark 4.0+. Verified locally against the Spark 4.0 profile.


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Re: [PR] feat: support dayname and monthname natively [datafusion-comet]

["mbutrovich (via GitHub)" <gi...@apache.org>]

mbutrovich merged PR #4544:
URL: https://github.com/apache/datafusion-comet/pull/4544


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