feat: support timestampadd and timestampdiff via codegen dispatch

[andygrove]

Which issue does this PR close?

There is no dedicated issue. This is part of the ongoing effort to provide native support for expressions that currently force a full fallback to Spark. Related to the expression coverage epic #240.

Rationale for this change

timestampadd and timestampdiff have no Comet handler today, so any query using them falls the entire operator back to Spark. Both are regular expressions (present since Spark 3.3), not RuntimeReplaceable, so they reach serde directly.

Rather than a native implementation, this PR routes them through the JVM codegen dispatcher. The dispatcher runs Spark's own generated code inside the native Comet pipeline, which keeps the operator native while guaranteeing bit-for-bit Spark compatibility across all supported versions. This avoids the datetime edge-case divergences (timezone and calendar handling) that a chrono-based native implementation would be prone to.

What changes are included in this PR?

• CometTimestampAdd and CometTimestampDiff codegen-dispatch serdes in datetime.scala, registered in QueryPlanSerde's temporalExpressions map.
• Comet SQL file tests timestampadd.sql and timestampdiff.sql covering all time units, negative quantities, month-end and leap-day rollover, whole-unit truncation, and NULL inputs.
• Support status for both functions added to the Supported Spark Expressions guide.

make_interval was considered but left for a follow-up: its output type is CalendarIntervalType, which Comet's columnar layer does not yet support (the dispatcher reports unsupported output type CalendarIntervalType and falls back). It can be enabled once CalendarIntervalType support lands (#4898).

How are these changes tested?

New Comet SQL file tests run each query through both Spark and Comet and verify the results match and that Comet executes the expression natively (through the dispatcher) rather than falling back. Coverage includes every supported time unit, positive and negative quantities, month-end and leap-day boundaries, whole-unit truncation toward zero, and NULL inputs.

[peterxcli]

make_interval was considered but left for a follow-up: its output type is CalendarIntervalType, which Comet's columnar layer does not yet support (the dispatcher reports unsupported output type CalendarIntervalType and falls back). It can be enabled once CalendarIntervalType support lands (#4898).

I think you're refer to #3099? I saw the native SparkMakeInterval implementation is ready and registered as comet udf, what we need to do is adding a new catalyst mapping classOf[MakeInterval] -> CometMakeInterval and CometMakeInterval scala implementation to convert expr to make_interval proto?

[peterxcli]

LGTM, only need to merge upstream and resolve conflict in expressions.md.

ai review: non-blocker review:

1. add MILLISECOND to both tests and MICROSECOND to timestampdiff.
2. Add one TIMESTAMP_NTZ or DST-boundary case; current tests only exercise timestamp-LTZ away from DST transitions.