feat(services/s3): support source-side conditional copy

[YuangGao]

Which issue does this PR close?

Closes #7090.

Rationale for this change

Destination-side If-Match protects the destination from being overwritten. Source-side x-amz-copy-source-if-* is the other half: it lets callers gate a copy on the source object's state, closing the race between stat(src) and copy(src, dst).

S3 CopyObject accepts four source-side conditions, mirroring the standard HTTP conditional set already exposed on OpStat / OpRead:

• x-amz-copy-source-if-match (ETag)
• x-amz-copy-source-if-none-match (ETag)
• x-amz-copy-source-if-modified-since (HTTP-date)
• x-amz-copy-source-if-unmodified-since (HTTP-date)

What changes are included in this PR?

Core:

• Add Capability::copy_with_source_if_{match,none_match,modified_since,unmodified_since}.
• Add the four matching fields to OpCopy / CopyOptions, builders on FutureCopy. ETag fields use Option<String>, time fields use Option<Timestamp> (same type already used by OpStat / OpRead).
• Add the four matching checks in correctness_check.rs and the capability-check layer.

S3:

• Set the four new capability flags to true on the S3 capability.
• s3_copy_object and s3_complete_multipart_copy inject the four x-amz-copy-source-if-* headers when set.

Tests:

• test_copy_with_source_if_match_match / _mismatch and the symmetric test_copy_with_source_if_none_match_* pair in core/tests/behavior/async_copy.rs. Time-based conditions share the same plumbing and are covered by the capability + correctness checks.

Are there any user-facing changes?

Yes, additive only — op.copy_with(from, to).source_if_match(etag) (and the three siblings) are new opt-in builders. Existing op.copy(from, to) is unchanged.

AI Usage Statement

AI-assisted implementation.