[Feature] Generating changelogs with Python writers for input changelog-producer

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/paimon/issues) and found nothing similar.

### Motivation

Currently the Python write path never produces changelog files. CommitMessage has no changelog field, DataWriter writes only one file, and FileStoreCommit creates snapshots with changelog_manifest_list=None.

### Solution

Implement INPUT mode only (double-write at flush time). FULL_COMPACTION and LOOKUP require compaction infrastructure that doesn't exist in Python at the moment.

### Anything else?

_No response_

### Are you willing to submit a PR?

- [x] I'm willing to submit a PR!

Start by tracing CommitMessage, DataWriter, and FileStoreCommit to understand the current Python write and snapshot paths. Done means INPUT mode double-writes changelog files at flush time and snapshots populate the changelog manifest; FULL_COMPACTION and LOOKUP remain out of scope.

Written by the indexing model from the issue text.