feat(storage): apply implicit file:// resolution to all top-level URI helpers

[achille-roussel]

Summary

• PR feat(storage): treat bare paths as file:// in LoadBucket #91 wired resolveImplicitFileURI into LoadBucket so storage.LoadBucket(ctx, "some-path") would treat a bare path as a file:// URI. The intent was for the same to hold for every top-level URI-taking helper, but the fix was only wired into one entry point — GetObject(ctx, "input.json") still failed with bucket not found.
• This PR applies resolveImplicitFileURI at the top of every *At helper: HeadObjectAt, GetObjectAt, PutObjectAt, DeleteObjectAt, CopyObjectAt (both sourceURI and destURI), and ListObjectsAt. The non-At wrappers and PutObjectAtWriter inherit the fix transitively.
• Two TestCopyObject subtests (invalid source URI, invalid dest URI) asserted that the bare string "invalid-uri" should fail; that's exactly the behavior being changed. The remaining bucket not found subtest still covers unknown-scheme rejection.

Why a per-function fix (not deeper)

• Inside registryFunc.LoadBucket: each *At helper splits the object URI before loading the bucket. For GetObject(ctx, "input.json"), uri.Split returns ("", "", "input.json"), so the call into registry.LoadBucket is already passing "". Resolution at the registry level is too late.
• Inside uri.Split: would change semantics for every caller (Split("foo") flipping from ("", "", "foo") to ("file", "", "/abs/cwd/foo")). uri.Split is exported and consumed elsewhere; broadening its file-path detection past the existing /, ./, ../, ~ rule is a load-bearing change not worth taking on here.

Test plan

• go test ./storage/...
• go test -race ./storage/
• New TestImplicitFileObjectURIs exercises Put → Head → Get → Copy → List → Delete with bare-path URIs under t.Chdir(t.TempDir()).
• Existing TestLoadBucketImplicitFile, TestResolveImplicitFileURI, and full TestStorage matrix still pass (no behavior changes for explicit URIs — resolveImplicitFileURI short-circuits on any input containing :).

Downstream

Once this lands, stripes's resolveSourceURI helper (which exists only to work around this asymmetry) becomes redundant and can be removed.

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