feat: impl MultipartStore for PrefixStore

[ddupg]

Which issue does this PR close?

Closes #586

Rationale for this change

PrefixStore currently lacks MultipartStore support, preventing use of low-level multipart APIs under a prefixed store. Implementing this aligns prefix semantics across both ObjectStore and MultipartStore .

What changes are included in this PR?

• Implement MultipartStore for PrefixStore<T> with T: MultipartStore , delegating calls after applying the prefix.
• Loosen generic constraints: PrefixStore no longer requires T: ObjectStore at the type definition; trait impls add bounds per trait ( ObjectStore impl requires T: ObjectStore , MultipartStore impl requires T: MultipartStore ).
• Add integration-style tests using existing multipart cases with PrefixStore<InMemory> .

Are there any user-facing changes?

• The generic bound on PrefixStore is relaxed (broader type), enabling use in contexts where only MultipartStore is needed. This is non-breaking.
• No API removals; behavior is enabled when the inner store implements the corresponding trait.