[PR] fix(core): resolve Delta partition values by storage name when column mapping is enabled [incubator-xtable]

["Mola-maker (via GitHub)" <gi...@apache.org>]

Mola-maker opened a new pull request, #973:
URL: https://github.com/apache/incubator-xtable/pull/973

   Fixes #972
   
   ## What is the purpose of the pull request
   
   With `delta.columnMapping.mode=name`, Delta writes the `partitionValues` of add/remove actions keyed by each column's physical name (`col-<uuid>`), while XTable's internal partition fields carry the logical name. `DeltaKernelPartitionExtractor` and `DeltaPartitionExtractor` looked values up by the logical name only, so every identity partition value came out `null` — the sync reported SUCCESS while the converted Iceberg table had NULL partition columns (e.g. `WHERE country='USA'` matched 0 rows instead of 2).
   
   Two gaps combined into the bug on the Kernel path: `DeltaKernelSchemaExtractor` computed each field's physical (storage) name from `delta.columnMapping.physicalName` but never stored it on the built `InternalField` (the Spark-based `DeltaSchemaExtractor` does set it), and neither partition extractor consulted `InternalField.getStorageName()` when resolving partition values.
   
   ## Brief change log
   
   - `DeltaKernelSchemaExtractor`: set the already-computed storage name on the built `InternalField`, mirroring `DeltaSchemaExtractor`.
   - `DeltaKernelPartitionExtractor` / `DeltaPartitionExtractor`: in `getSerializedPartitionValue`, resolve identity partition values by the field's storage name first when one is present, falling back to the logical name.
   - Tests: added `testPartitionValueExtractionWithColumnMappingPhysicalName` to `TestDeltaKernelPartitionExtractor` and `TestDeltaPartitionExtractor`, feeding a partition-values map keyed by the physical name.
   
   ## Verify this pull request
   
   This change added tests and can be verified as follows:
   
   - Before the fix, the new extraction test fails: expected partition range `USA`, actual `null`. Verified in CI (fork run of this branch without the fix): xtable-core ran 550 tests with this single failure.
   - After the fix, the same CI build (`Maven CI Build` on the fork PR) passes: xtable-core `Tests run: 550, Failures: 0, Errors: 0, Skipped: 3`, BUILD SUCCESS. Red/green runs: https://github.com/Mola-maker/incubator-xtable/pull/1
   - `./mvnw -pl xtable-core -Dtest=TestDeltaKernelPartitionExtractor,TestDeltaPartitionExtractor test`


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