branch-4.1: [feature](paimon) Paimon write support via JNI

[suxiaogang223]

Summary

Implement Paimon table write support (INSERT INTO / INSERT OVERWRITE) for Doris via JNI, bridging to the Paimon Java SDK. The architecture uses a single-writer model (analogous to Iceberg) with partition and bucket routing delegated to the Paimon SDK.

Architecture

VPaimonTableWriter (single AsyncResultWriter)
  └── IPaimonWriteBackend
        ├── JniPaimonWriteBackend  ← Java SDK via JNI
        └── FfiPaimonWriteBackend  ← stub for future Rust backend

Changes

Layer	Description
Thrift	TPaimonTableSink, TPaimonCommitMessage
BE	IPaimonWriteBackend / IPaimonWriter / IPaimonCommitter interfaces; JNI backend; FFI stub; VPaimonTableWriter; Pipeline Operator
FE	Nereids planner integration; PaimonTransaction; PaimonInsertExecutor
Java	PaimonJniWriter (Arrow IPC → Paimon SDK)

Not Included

• Paimon DDL (CREATE TABLE) — tables created externally
• Compaction, Lookup/FullCompaction Changelog Producer
• Rust FFI backend

[hello-stephen]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[suxiaogang223]

run buildall

[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 3.62% (16/442) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	72.88% (28598/39238)
Line Coverage	56.38% (309604/549162)
Region Coverage	53.61% (258791/482765)
Branch Coverage	54.57% (113606/208184)