Deduplicate Throwable cause-chain traversal in Comet tests

[peterxcli]

What is the problem the feature request solves?

Several Comet test suites independently implement the same Throwable.getCause traversal:

• CometTemporalExpressionSuite defines causeChain for structured Spark exception checks added in fix: surface next_day and make_date ANSI errors as Spark exceptions #5167.
• CometRegExpJvmSuite walks the chain to inspect exception class names.
• ParquetReadSuite walks the chain to find SchemaColumnConvertNotSupportedException.
• CometExecSuite contains two copies that inspect exception messages.
• CometExpressionSuite defines arithmeticError, which independently walks the chain to find a SparkArithmeticException.

This duplicates null termination, ordering, and diagnostic behavior across six call sites in five suites.

Other code in CometExpressionSuite, CometDppFallbackRepro3949Suite, CometSQLQueryTestHelper, and CometCastSuite intentionally uses bounded, root-only, or one-level unwrapping semantics and should not be changed automatically.

Describe the potential solution

Add one protected causeChain(error: Throwable): Seq[Throwable] helper to CometTestBase and migrate these cause-chain walks to it.

Keep deepestSparkThrowable local to CometTemporalExpressionSuite because selecting the deepest structured Spark exception is currently unique policy; it can use the inherited causeChain helper. Do not introduce a universal structured-exception comparator or change tests that intentionally inspect only one cause level.

Validate that the migrated tests preserve outer-to-inner ordering and behavior across supported Spark versions.

Additional context

Raised while addressing review feedback on #5167.

[peterxcli]

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