[fix](nereids) Fix DST spring-forward gap handling in datetime literal timezone conversion

[Mryange]

What problem does this PR solve?

Issue Number: N/A

Problem Summary:

When inserting a TIMESTAMPTZ value whose civil time falls in a DST spring-forward gap
(e.g. '2024-03-10 02:30:00 America/New_York' — a time that does not exist because
clocks jump from 02:00 EST to 03:00 EDT), the two FE literal-parsing paths produced
different UTC instants:

Path	Input	Stored (UTC)
Named-TZ suffix ('... America/New_York')	02:30:00 NY	❌ 06:30 UTC
Implicit session TZ (SET time_zone='America/New_York')	02:30:00	✅ 07:30 UTC

The correct answer is 07:30 UTC: Java's ZonedDateTime.of() snaps the non-existent
gap time to 03:30 EDT (= 07:30 UTC), but the old code then queried
zoneId.getRules().getOffset(thatTime) which returned the post-snap EDT offset
(-04:00) instead of the pre-gap EST offset (-05:00). Applying this wrong offset
to the original pre-snap civil time 02:30 yielded the incorrect 06:30 UTC.

Root cause

Both parseDateTimeLiteral() and init() in DateTimeLiteral.java used the pattern:

Instant thatTime = ZonedDateTime.of(civil, zoneId).toInstant();   // correct snap
int offset = sessionTz.getOffset(thatTime) - zoneId.getOffset(thatTime); // wrong: uses post-snap offset
// ... apply offset to pre-snap civil fields → wrong result

Fix

Replace the offset-delta arithmetic with a direct zone projection of the correctly
snapped absolute instant:

Instant thatTime = ZonedDateTime.of(civil, zoneId).toInstant(); // correct snap
ZonedDateTime inTarget = thatTime.atZone(targetZone);           // project, don't compute delta
// read fields from inTarget

This applies to both the session-timezone path (for DateTimeLiteral) and the
UTC-storage path (for TimestampTzLiteral).

Changes

• fe/fe-core/src/main/java/org/apache/doris/nereids/trees/expressions/literal/DateTimeLiteral.java:
  Fix parseDateTimeLiteral() and init() to convert via Instant.atZone() instead
  of an offset delta.
• fe/fe-core/src/test/java/org/apache/doris/nereids/trees/expressions/literal/TimestampTzLiteralTest.java:
  Add testSpringForwardGapConsistency() unit test.
• regression-test/suites/datatype_p0/timestamptz/test_timestamptz_dst_gap.groovy:
  Add regression test covering both named-TZ and implicit session-TZ paths for a
  spring-forward gap time, asserting they produce the same UTC instant.

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[hello-stephen]

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