[lake/lance] Add tests for Decimal and Timestamp(LTZ) conversions

[XuQianJin-Stars]

Extend Lance test coverage for Decimal and Timestamp/TimestampLtz mappings on both the Fluss RowType -> non-shaded Arrow schema layer (LanceArrowUtils) and the shaded -> non-shaded VectorSchemaRoot copy layer (ArrowDataConverter):

• LanceArrowUtilsTest: assert Arrow type for DECIMAL(9,2) / DECIMAL(20,4) including precision and scale round-trip; assert TimeUnit selection for TIMESTAMP_LTZ(0/3/6/9) and TIMESTAMP(0/3/6/9).
• ArrowDataConverterTest: end-to-end conversion of DECIMAL(20,4) values through ShadedArrowBatchWriter into a non-shaded DecimalVector, and TIMESTAMP_LTZ(3) / TIMESTAMP_LTZ(6) columns into TimeStampMilliVector and TimeStampMicroVector respectively, including sub-millisecond precision truncation semantics.

These types are common in financial and event-time workloads; adding explicit coverage guards against future regressions in the Lance tiering path.

Purpose

Lance tiering already handles DECIMAL, TIMESTAMP and TIMESTAMP_LTZ in production code, but the existing unit tests did not directly exercise:

1. The precision/scale round-trip for DECIMAL on the shaded -> non-shaded schema mapping,
2. The TimeUnit selection rules (SECOND / MILLISECOND / MICROSECOND / NANOSECOND) for TIMESTAMP and TIMESTAMP_LTZ at every supported precision (0/3/6/9),
3. The end-to-end value conversion from Fluss Decimal / TimestampLtz through ShadedArrowBatchWriter into the corresponding non-shaded Arrow vectors (including sub-millisecond truncation for TIMESTAMP_LTZ(3) and microsecond preservation for TIMESTAMP_LTZ(6)).

This PR closes that gap so that any future refactor of LanceArrowUtils or ArrowDataConverter will surface regressions on these two commonly-used-in-production types immediately.

Linked issue: close #xxx

Brief change log

Test-only change. No production code is touched.

• LanceArrowUtilsTest: add three tests (+65 lines)
  • testToArrowSchemaWithDecimal — asserts ArrowType.Decimal(9,2) and ArrowType.Decimal(20,4) are produced for the corresponding Fluss types.
  • testToArrowSchemaWithTimestampLtz — asserts TIMESTAMP_LTZ(0/3/6/9) maps to Arrow Timestamp with SECOND / MILLISECOND / MICROSECOND / NANOSECOND.
  • testToArrowSchemaWithTimestampNtz — same coverage for TIMESTAMP(0/3/6/9) (no time zone).
• ArrowDataConverterTest: add three tests (+104 lines)
  • testConvertDecimalColumn — writes three DECIMAL(20,4) values (positive / negative / very small) via ShadedArrowBatchWriter, converts to a non-shaded VectorSchemaRoot, and verifies each value using BigDecimal.compareTo on DecimalVector#getObject.
  • testConvertTimestampLtzMillisColumn — writes three TIMESTAMP_LTZ(3) values built with TimestampLtz.fromEpochMillis and asserts round-trip via TimeStampMilliVector#get.
  • testConvertTimestampLtzMicrosColumn — writes TIMESTAMP_LTZ(6) values with sub-millisecond components (Instant.ofEpochMilli(...).plusNanos(...)) and asserts the expected microsecond value in TimeStampMicroVector (i.e. millis * 1_000 + nanoOfMillis / 1_000), locking in the truncation semantics.

Tests

New tests are the deliverable of this PR. Verified locally:

• ./mvnw -pl fluss-lake/fluss-lake-lance test -Dspotless.check.skip — all Lance tests pass, including the 6 new ones.
• ./mvnw -pl fluss-lake/fluss-lake-lance checkstyle:check — 0 violations.

No production behavior change; existing tests are unchanged.

API and Format

None. This PR only adds tests under src/test/java. There are no changes to public Java API, table properties, wire format, or on-disk / Lance dataset format.

Documentation

None. Test-only change; no user-facing documentation impact.