[MINOR][CI] Add new Delta known-failures caused by Velox decimal cast update

[felipepessoto]

What changes were proposed in this pull request?

The Delta-on-Gluten CI gate (Delta Spark UT pipeline) started reporting new regressions (test failures not previously in known-failures.txt) after the daily Velox version bump in #12769, which pulled in upstream Velox commit a96d0415c — "fix(spark): Enable ANSI-compliant cast from DECIMAL to DECIMAL". This changed DECIMAL cast/overflow/widening behavior.

This PR adds the 4 newly-failing tests to known-failures.txt to unblock CI while the underlying behavior change is investigated further.

org.apache.spark.sql.delta.ImplicitMergeCastingSuite#MERGE overflow in WHEN MATCHED THEN UPDATE SET t.value = s.value sourceType: DECIMAL(3,1), targetType: DECIMAL(3,2) followAnsiEnabled: false, ansiEnabled: true, storeAssignmentPolicy: LEGACY
org.apache.spark.sql.delta.ImplicitStreamingMergeCastingSuite#Streaming MERGE overflow sourceType: DECIMAL(3,1), targetType: DECIMAL(3,2) followAnsiEnabled: false, ansiEnabled: true, storeAssignmentPolicy: LEGACY
org.apache.spark.sql.delta.typewidening.TypeWideningInsertSchemaEvolutionBasicSuite#INSERT - never automatic type widening DECIMAL(9,2) -> DECIMAL(18,2)
org.apache.spark.sql.delta.typewidening.TypeWideningInsertSchemaEvolutionBasicSuite#INSERT - never automatic type widening DECIMAL(9,2) -> DECIMAL(19,3)

Why are the changes needed?

Mitigate #12779

Does this PR introduce any user-facing change?

No. This only updates the CI known-failures allowlist.

How was this patch tested?

N/A — CI configuration/test-allowlist change only. Verified the failing test names against the scheduled run https://github.com/apache/gluten/actions/runs/31866072615#summary-94980767356:

• https://github.com/apache/gluten/actions/runs/31866072615/job/94969534448
• https://github.com/apache/gluten/actions/runs/31866072615/job/94969534423
• https://github.com/apache/gluten/actions/runs/31866072615/job/94969534400

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Pull request overview

Updates the Delta-on-Gluten CI gate allowlist to account for newly introduced Delta Spark UT failures after the Velox daily bump (notably the DECIMAL→DECIMAL ANSI cast behavior change), unblocking the Delta Spark UT pipeline while the regression is investigated (ref: #12779).

Changes:

• Added 4 newly failing Delta Spark tests to the Delta Spark UT known-failures.txt baseline.

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