branch-4.1: [fix](multi-catalog) Preserve external partition metadata

[suxiaogang223]

What problem does this PR solve?

Issue Number: None

Related PR: #62821, #65581, #66010

Problem Summary: branch-4.1 already backported the Paimon and BE partition metadata changes through #65581, but Hive-style scans, Hudi, Iceberg, and file load paths still had incomplete FE metadata propagation. Hive default partitions were encoded as \N, which lost the distinction between NULL and a literal \N. Iceberg attached partition metadata only when runtime partition pruning was enabled, used unstable per-map ordering, and did not safely handle files written with different partition specs.

This change:

• propagates aligned partition values and explicit NULL flags through Hive-style scans, Hudi, and both legacy and Nereids file load paths;
• preserves a literal \N as data while representing Hive default partitions as NULL;
• sends stable Iceberg identity-partition metadata for every split, keyed by both spec ID and partition data;
• restricts Iceberg path partition keys to identity columns shared by all partition specs, which is safe for both FileScannerV2 and the legacy scanner's scan-level slot mapping.

Release note

Fix partition-column materialization, NULL handling, and Iceberg partition evolution compatibility for external table scans and file loads on branch-4.1.

Check List (For Author)

• Test: Unit Test
  • ./run-fe-ut.sh --run org.apache.doris.common.util.BrokerUtilTest,org.apache.doris.datasource.hive.source.HiveScanNodeTest,org.apache.doris.datasource.iceberg.IcebergUtilsTest,org.apache.doris.datasource.iceberg.source.IcebergScanNodeTest,org.apache.doris.datasource.paimon.source.PaimonScanNodeTest
• Behavior changed: Yes. External partition metadata and NULL values are materialized consistently per split.
• Does this need documentation: No

[hello-stephen]

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1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
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[suxiaogang223]

run buildall

[hello-stephen]

FE UT Coverage Report

Increment line coverage 52.48% (53/101) 🎉
Increment coverage report
Complete coverage report

[hello-stephen]

FE Regression Coverage Report

Increment line coverage 71.29% (72/101) 🎉
Increment coverage report
Complete coverage report

[suxiaogang223]

run buildall

[hello-stephen]

FE Regression Coverage Report

Increment line coverage 70.71% (70/99) 🎉
Increment coverage report
Complete coverage report

[suxiaogang223]

run buildall

[hello-stephen]

FE UT Coverage Report

Increment line coverage 53.70% (58/108) 🎉
Increment coverage report
Complete coverage report

[hello-stephen]

FE Regression Coverage Report

Increment line coverage 21.30% (23/108) 🎉
Increment coverage report
Complete coverage report