fix(mem_wal): stream conditional file promotion

[jackye1995]

Summary

• stream fixed-name MemWAL promotions through the client on native S3 instead of using server-side copy
• preserve writer fencing with conditional single-part and multipart destination creation
• retain the existing native conditional-copy path for GCS, Azure, local, OpenDAL, and custom stores
• treat an equal-size existing destination as an idempotent retry without overwriting it
• pin Add PutMultipartOptions::mode for conditional multipart completion apache/arrow-rs-object-store#863 until its conditional multipart API is released

Validation

A full 1,161,228-row LAION-1M Blob v2 MemWAL campaign completed on S3 with one discarded prewarm and one measured trial. The measured run completed at 626.9 rows/s, passed all 101 key and image checksum samples, and completed the formerly failing 3.49-GiB conditional SSTable promotion without CopyObject or UploadPartCopy.

[claude]

Claude Code Review

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[lance-gatekeeper]

✅ Gate recommendation: approve.

The backend fallback and wrapper fix address both reported regressions. Native S3 can use conditional streaming while unverified stores retain their established conditional-copy path. No remaining blockers found.