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.

Bắt đầu với MaintenanceTable và ManifestWriterV2, sau đó kiểm tra các phần nội bộ được tham chiếu của write_manifest_list và việc cập nhật snapshot. Sử dụng các quy tắc về chuỗi theo từng phân vùng được nêu và ma trận kiểm thử phân loại làm tiêu chí chấp nhận; được xem là hoàn tất khi một commit thay thế chỉ chứa metadata chỉ xóa các mục xóa mồ côi, đồng thời giữ lại các mục không rõ ràng và đang hoạt động.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.