feat(xio): make copy buffer size configurable

[localai-bot]

Description

Adds functional options to xio.Copy and a WithBufferSize option. Copies use a 1 MiB buffer by default while existing callers remain source compatible. Non-positive overrides fall back to the default.

This addresses severe progress-callback amplification during large model imports. The downloader reports progress after every copy chunk. In distributed mode, each report synchronously updates gallery_operations in PostgreSQL and publishes progress through NATS. The previous 32 KiB chunks can therefore turn one large import into hundreds of thousands of database updates and messages.

A live 10-second sample during a slow import recorded 250 NATS input messages and 244 PostgreSQL row updates. At 32 KiB per callback, 25 callbacks per second corresponds to 0.78 MiB/s, matching the observed import rate. A 1 MiB buffer reduces callback frequency by about 32x.

A separate production-shaped NAS probe showed similar raw SMB write throughput for both buffer sizes: 32 KiB averaged 71.7 MiB/s and 1 MiB averaged 72.3 MiB/s. This confirms that callback amplification, rather than CIFS write coalescing, is the bottleneck.

Notes for Reviewers

• go test ./pkg/xio ./pkg/downloader ./pkg/oci
• go test ./pkg/xio -run "^$" -bench BenchmarkCopy -benchtime=1x
• Added Ginkgo coverage for complete copies, default and overridden sizes, invalid-size fallback, and cancellation.
• Added benchmarks for the default, 32 KiB, 1 MiB, and 4 MiB buffers.

Signed commits

• Yes, I signed my commits.
• Documentation updated (docs/content/) for user-facing changes, or not applicable