Revisit CometCastSuite assumptions now that Cast has a codegen dispatch fallback

[andygrove]

What is the problem the feature request solves?

CometCastSuite was written when a cast was either implemented natively or fell back to Spark
for the whole plan. CometCast now mixes in CodegenDispatchFallback, so an Unsupported
support level no longer means the plan leaves Comet -- the JVM codegen dispatcher runs Spark's
own generated code for the cast inside the Comet pipeline, and the results are
Spark-compatible by construction.

Several assumptions baked into the suite no longer hold:

1. all valid cast combinations covered conflates "unsupported" with "broken". The
   assertion requires that any pair where CometCast.isSupported is not Compatible must have
   an ignored test, on the grounds that the test would otherwise fail. For a
   codegen-dispatched cast the test would now pass, so the assertion forces us to ignore
   working coverage. See the cast BooleanType to DecimalType(10,2) case added in
   feat: remove native cast from boolean to decimal #5185, where the test is ignored purely to
   satisfy this rule while the DecimalType(14,4) and DecimalType(30,0) variants stay active
   and pass.

2. Existing ignored tests may be stale. Many are ignored with comments about wrong results
   or Arrow errors from the native path (for example the cast TimestampType to ByteType family).
   Those casts are now dispatched rather than executed natively, so some of them likely produce
   correct results today. Nobody has re-checked them since the dispatcher landed.

3. castTest's hasIncompatibleType flag is ambiguous. It currently means "do not assert
   native execution". With dispatch, a cast can be non-native in the sense of having no Rust
   kernel while still keeping the plan fully inside Comet, and the flag cannot express that
   distinction.

Describe the potential solution

Roughly, in increasing order of effort:

• Teach all valid cast combinations covered about the three outcomes we now care about --
  native, codegen-dispatched, and genuine Spark fallback -- instead of the current
  compatible/ignored binary, so a dispatched cast can keep an active test.
• Audit the ignored tests and un-ignore the ones that now pass through the dispatcher, updating
  or deleting the stale comments and issue links.
• Split hasIncompatibleType into distinct expectations for "runs natively", "runs in Comet via
  the dispatcher", and "falls back to Spark".

Additional context

Context in #5185.