Add maintenance action to remove dangling delete files

### 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.

从 MaintenanceTable 和 ManifestWriterV2 开始，然后检查所引用的 write_manifest_list 和 snapshot 更新内部实现。使用所述的按分区序列规则和分类测试矩阵作为验收标准；完成意味着纯元数据替换提交只移除悬空删除，同时保留有歧义的条目和活动条目。

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