Add maintenance action to remove dangling delete files

MaintenanceTable と ManifestWriterV2 から始め、次に参照されている write_manifest_list と snapshot 更新の内部実装を調査します。指定されたパーティションごとのシーケンス規則と分類テストマトリクスを受け入れ基準として使用します。完了とは、メタデータのみを置き換えるコミットが孤立した削除だけを削除し、曖昧なエントリとライブエントリを保持することです。

Feature Request / Improvement

Streaming upsert writers (e.g. AWS Firehose Iceberg delivery) write equality-delete files on every commit. Compaction applies them into rewritten data files but leaves the entries in the manifests, and expire_snapshots can't touch files the current snapshot still references — so they accumulate without bound. On one of our production tables we measured ~90K dangling delete entries growing ~4.6K/day, and every query planning over recent partitions has to read the ever-growing delete manifests.

Java Iceberg handles this (rewrite_data_files with remove-dangling-deletes, RemoveDanglingDeletesSparkAction), but PyIceberg's MaintenanceTable currently only has expire_snapshots, and engines like Athena expose no statement for it either — so users on Athena/Firehose stacks have no non-Spark way out.

Proposal: table.maintenance.remove_dangling_deletes() — a metadata-only commit that:

• classifies per (partition_spec_id, partition): an equality delete at sequence s is dangling iff no live data file in that partition has sequence < s (position deletes: <= s); ambiguous cases (unpartitioned specs, unknown content) are kept
• carries data manifests through unchanged, drops fully-dangling delete manifests, rewrites mixed ones to their surviving entries, and commits as a replace snapshot against the current ref

One enabler is worth a small standalone fix first: ManifestWriterV2 hardcodes content=data, so PyIceberg currently can't write delete-content manifests at all.

We have a working implementation built on PyIceberg 0.12 internals (write_manifest_list, a ManifestWriterV2 subclass, AddSnapshotUpdate/SetSnapshotRefUpdate with AssertRefSnapshotId), validated against production Glue/Athena tables, with a test matrix for the classification rules. Happy to contribute it if there's interest.