[Python] Keep REST commits with callbacks on the native path

[JingsongLi]

Why

PyPaimon supports commit callbacks, but registering a callback forces REST commits to use the Python implementation even when Native commit is enabled.

Changes

• Register the existing callbacks through Native's public add_commit_callback interface and keep eligible REST commits on the Native publication path.
• Adapt the typed Native CommitCallbackContext to the existing Python CommitCallbackContext, including Python Snapshot, ManifestEntry, GenericRow, canonical file paths and assigned row-tracking metadata.
• Decode one context per notification and share it across callbacks in registration order. Callbacks added after Native initialization or between commits take effect on later commits.
• Preserve the existing callback close lifecycle, propagate callback exceptions, and retain committed files after notification failures or lost publication responses.

Publication and recovery notification belong to Rust core; Python adapts the existing API. The public Python callback API is unchanged.

Verification

• Native commit / write / existing callback regression suites: 187 passed.
• Native counters: 127 plans, 115 reads, 86 writes and 88 commits exercised.
• Coverage includes append, primary-key and Data Evolution tables; overwrite ADD/DELETE entries; empty commits; lost REST responses; callback errors; Native reads inside callbacks; late registration; shared contexts; close errors and callback-owner GC cycles.
• Flake8 and Python 3.6 grammar validation for changed Python sources: passed.

Dependency

Requires apache/paimon-rust#1108. This PR is draft until the Rust change merges; Native CI continues building upstream Rust main.

[leaves12138]

LGTM from the code review. The native callback bridge preserves the existing Python context, callback ordering/shared context, late registration, recovery, error propagation, and close/GC lifecycle. The Python Native CI failures are expected until dependency PR apache/paimon-rust#1108 is merged: that job currently builds Rust main, so _add_commit_callback is unavailable and the new tests intentionally fail on Python fallback. Please rerun Native CI after #1108 lands.