[SPARK-40008][SQL][FOLLOWUP] Fix codegen of casting integrals to ANSI intervals

[MaxGekk]

What changes were proposed in this pull request?

This is a follow up of #37442 that fixes the codegen of casting integrals to ANSI intervals w/ multi units. The PR removes the assert:

assert(it.startField == it.endField)

Why are the changes needed?

To make codegen consistent to interpreted code, and fix the assert.

Does this PR introduce any user-facing change?

Yes.

Before:

> select cast(dt as interval hour to second) from values(100Y) as t(dt);
java.lang.AssertionError
assertion failed

After:

> select cast(dt as interval hour to second) from values(100Y) as t(dt);
0 00:01:40.000000000

How was this patch tested?

By running new tests:

$ build/sbt "sql/testOnly org.apache.spark.sql.SQLQueryTestSuite -- -z cast.sql"

[MaxGekk]

The failure of ImageFileFormatSuite is not related to the changes. @HyukjinKwon @cloud-fan @gengliangwang Could you take a look at the PR, please.

[MaxGekk]

Merging to master. Thank you, @cloud-fan for review.