[JingsongLi]

Summary

• Adapt normalized PyPaimon MERGE branches to SDK WhenMatched/WhenNotMatched and MergeAssignment objects for both existing table MERGE and distributed batch execution.
• Use the named for_updates, for_deletes, and for_inserts factories. Restore requested deletion schema metadata at the Arrow boundary.
• Preserve public PyPaimon MERGE APIs/types, ordered conditions, ON-key aliases, callable views, source handling, serialization and backend isolation.
• Replace test dictionary transport expectations with typed SDK doubles and cover batch/stream table MERGE, mixed actions, all assignment forms and deletion schemas.

No public PyPaimon API or dependency changes. Native execution remains a thin adapter and has no Python executor dependency. No version probes or old protocol compatibility paths are added.

Verification

• 105 focused protocol, public MERGE, metadata and execution-boundary tests passed; 16 Native cases skipped in the regular Python environment.
• Arrow 16 compatibility: 35 passed, 1 skipped.
• 156 real SDK/PyPaimon Native integration cases passed against the companion Rust implementation inside a Cargo PyO3 test executable, including the skipped MERGE/source-metadata cases and real batch execution adapters. No Native extension was installed or rebuilt into the Python environment; the temporary cross-repository runner is not committed.
• Scoped flake8 and diff checks passed. Independent cross-review found no remaining actionable regressions.

Dependency

Depends on apache/paimon-rust#1204. This PR remains draft until that API reaches Rust main. Native CI continues to build main.

[leaves12138]

LGTM