fix: scope fallback reasons to the current conversion - #7095

Which issue does this PR close?

Closes #5499.

Rationale for this change

Stale fallback reasons on shared expression nodes can be lifted onto unrelated expressions and operators. These reasons can also satisfy strict fallback checking, hiding a conversion that failed without recording an explanation.

What changes are included in this PR?

• Track fallback reasons in a conversion-scoped journal keyed by node identity.
• Use current conversion records for expression copy-back and operator roll-up, including Spark 4.x shim rewrites.
• Preserve legitimate literal fallback reasons and operator reasons recorded by earlier planning rules.
• Restore the previous journal after nested conversions and exceptions, without sharing it with child threads.
• Add regression tests for stale and copied tags, decimal rewrites, shim failures, and journal isolation.

How are these changes tested?

• Spark 4.1: all 104 tests in CometExecRuleSuite passed.
• Spark 4.1: all 14 targeted tests passed across CometExecRuleSuite, CometWindowExecSuite, and CometDecimalPromotionSuite.
• Spark 3.4 with Java 17: all 11 targeted tests passed.
• Native build, Spotless, and Scalastyle passed.