test: rename CometCastSuite to CometNativeCastSuite

[andygrove]

Which issue does this PR close?

N/A — test/docs clarity.

Rationale for this change

CometCast mixes in CodegenDispatchFallback, so a cast that CometCast.isSupported rejects does not fall back to Spark — it still runs inside the Comet operator by way of the Arrow-direct codegen dispatcher, evaluating Spark's own Cast against Arrow vectors.

CometCastSuite only covers the first half of that: native cast parity, plus the isSupported matrix that decides which casts reach the native kernel. The name didn't convey the split, and reading a (from, to) pair asserted as Unsupported in that suite is easy to mistake for "this cast isn't tested" rather than "this cast is tested through the dispatcher instead".

What changes are included in this PR?

• Rename CometCastSuite → CometNativeCastSuite.
• Add a class-level scaladoc stating the scope explicitly: Spark-parity for the native Cast implementation and the isSupported matrix that gates it; codegen dispatch semantics are out of scope and belong to CometCodegenSuite and friends. It also notes that adding a native cast means moving a pair out of the Unsupported assertions and into the parity matrix.
• Annotate "cast ArrayType(DateType) to unsupported ArrayType routes through codegen dispatch" to say it is a boundary case — it only pins that Unsupported keeps the enclosing operator native — not dispatch coverage.
• Update every cross-reference to the old name: the expressions suite group in pr_build_linux.yml / pr_build_macos.yml, the -Dsuites= examples in development.md, a note in spark_configs_support.md, two SQL fixture headers under sql-tests/expressions/cast/, the CometCast.scala comment on the shared reason string, and two test comments in native/spark-expr/src/conversion_funcs/string.rs.

The historical entry in docs/source/changelog/0.1.0.md is left alone.

No test logic changed — this is a rename plus comments.

How are these changes tested?

./mvnw test -Dtest=none -Dsuites="org.apache.comet.CometNativeCastSuite valid" — suite is discovered under the new name, 3 tests run, all pass. ./mvnw test-compile, spotless:apply, scalastyle, cargo fmt --check, and prettier are all clean.

[andygrove]

@comphead hopefully this helps re your question in #5185 (comment)

[comphead]

Thanks @andygrove makes sense