[Feature]: Replace crossStorage with smarter auto-detection and clearer copyMode config

[huanghaoyuanhhy]

Part of #637 (Overly Complex Configuration File)

Is your feature request related to a problem? Please describe.

The current minio.crossStorage configuration is confusing and hard to understand for users:

1. The name describes the scenario, not the behavior. crossStorage sounds like "are you doing cross-storage backup?" but it actually controls how data is copied — via the storage system's native COPY API (false) or proxied through the backup server process (true). Users have to read a dedicated FAQ just to understand what it means (docs/user_guide/cross_storage.md).

2. Manual configuration is needed for same-type-different-instance scenarios. Auto-detection (core/backup/task.go:111) only compares Provider — if both are minio but at different addresses (Minio A → Minio B), the user must manually set crossStorage: true. This is the most confusing case: "both are Minio, why do I need crossStorage?"

3. Wrong config section. crossStorage is under the minio section, but it's not a MinIO setting — it's a backup behavior setting.

4. The FAQ in the docs is itself evidence of a naming failure. If a config option needs a dedicated FAQ entry explaining "what does this mean", the name isn't working.

Describe the solution you'd like.

This is a breaking change proposal:

1. Smarter auto-detection (P0) — eliminate manual configuration

Extend auto-detection to compare address and port in addition to provider:

// current: only compares provider
if backupProvider != milvusProvider { copyByServer = true }

// proposed: also compare address/port
if backupProvider != milvusProvider ||
   backupAddress != milvusAddress ||
   backupPort != milvusPort {
    copyByServer = true
}

This covers the Minio A → Minio B scenario automatically. Users almost never need to set this manually.

2. Rename the config (P1) — name should describe behavior, not scenario

Replace minio.crossStorage with an enum-based copyMode:

backup:
  copyMode: "auto"     # auto-detect (default, recommended)
  # copyMode: "direct" # force storage COPY API
  # copyMode: "proxy"  # force server-side proxy transfer

• auto: current implicit behavior, picks the right mode automatically
• direct: uses storage's native COPY API (fast, requires both ends accessible to each other)
• proxy: data flows through backup server (slower, works across any storage combination)

3. Move config from minio to backup section (P2)

crossStorage is not a storage configuration — it's a backup operation behavior. It belongs under the backup section.

4. Log the actual copy mode at task start (P3)

INFO  copy mode: proxy (auto-detected: backup and milvus storage have different addresses)
INFO  copy mode: direct (same storage, using native COPY API)

Describe an alternate solution.

A minimal non-breaking approach: keep crossStorage as-is but improve auto-detection (P0) so users rarely encounter it. Add deprecation warnings pointing to the new config name.

Anything else? (Additional Context)

Code references

• Config definition: internal/cfg/cfg.go:234,277
• Auto-detection: core/backup/task.go:110-113, core/check/task.go:139-150
• Copier selection: internal/storage/copier.go:126-137
• Restore skip logic: core/restore/coll_task.go:336-343
• Backup copy: core/backup/coll_dml_task.go:474-483
• User docs: docs/user_guide/cross_storage.md

Migration path

• Support both old (minio.crossStorage) and new config for one release cycle with a deprecation warning
• Remove the old config in the following release