[JingsongLi]

Summary

• Replace MERGE dictionary/tuple descriptions with immutable SDK MergeAssignment, WhenMatched, and WhenNotMatched objects.
• Replace the public MergeBatchExecution(..., clauses, mode) constructor with for_updates, for_deletes, and for_inserts; retain execute(Table | RecordBatch) -> Table.
• Reuse the same typed clause conversion for batch/stream merge_into and joined-batch execution, delegating selection and casts to existing Core logic.
• Preserve ordered first-match behavior, NULL fallthrough, callback grouping, lazy scalar conversion, original callback exceptions and exact output schemas. Delete factories retain every matched condition and skip UPDATE assignment conversion/evaluation.
• Make explicit literals authoritative even when their Python value is callable; keep Arrow array/chunked-array values supported.
• Document the factories and migrate SDK integration tests. Old dictionary descriptions and the old mode constructor are rejected. No dependencies are added.

Verification

• 13 embedded PyO3 MERGE tests passed, including strict clause families, mapping snapshots, lazy literals, delete-only evaluation, exception identity, concurrent callers and batch/stream source handling.
• 156 real SDK/PyPaimon Native integration cases passed across SDK update/Blob tests and PyPaimon MERGE/adapter/source-metadata tests. These ran against the real registered native module inside a Cargo test executable; no Native extension was installed or rebuilt into a Python environment. The temporary integration entry point is not included in this PR.
• Rust 1.98.0 all-targets Clippy with warnings denied, rustfmt and diff checks passed.
• Companion Python protocol/public-API checks: 105 passed, 16 skipped; Arrow 16: 35 passed, 1 skipped. Skipped Native cases were separately covered by the real-module integration run.

The companion Python PR apache/paimon#10764 remains draft until this SDK API is merged. Its Native CI continues to build Rust main; no old-SDK compatibility protocol is retained.

[leaves12138]

LGTM