fix(services/b2): percent-encode the object path in presigned requests

[PDGGK]

Which issue does this PR close?

None — found while reading the service.

Rationale for this change

The three presign arms interpolate the object path raw, while the ordinary read and write paths in the same crate encode it.

Presign (backend.rs), Stat and Read:

let url = format!(
    "{}/file/{}/{}?Authorization={}",
    auth_info.download_url, self.core.bucket, path, resp.authorization_token
);

Presign Write:

req = req.header("X-Bz-File-Name", build_abs_path(&self.core.root, path));

The reference, core.rs:

// :144-149  download_file_by_name
let url = format!("{}/file/{}/{}",
    auth_info.download_url, self.bucket, percent_encode_path(&path));

// :262  upload_file
req = req.header(X_BZ_FILE_NAME, percent_encode_path(&p));

Handing the formatted strings to http::Request::get shows what a presigned URL actually becomes:

path	resulting URI
dir/file.txt	path=/file/bkt/dir/file.txt, query=Authorization=TOK ✅
a#b.txt	path=/file/bkt/a, query=None
a?b.txt	path=/file/bkt/a, query=b.txt?Authorization=TOK
a b.txt	BUILD ERROR: invalid uri character
a%20b.txt	sends a%20b.txt, which B2 decodes to a b.txt

The # row is the worst: the URL is truncated to a different object and the entire ?Authorization= is swallowed as a fragment, so the presigned URL carries no token at all. a b.txt cannot be presigned even though op.read("a b.txt") on the same object works — that call goes through the encoding path. And the write header sends a%20b.txt where op.write("a%20b.txt") sends a%2520b.txt, so a presigned upload lands on a different key than an ordinary write for the same OpenDAL path.

Blast radius — the first row is the one that matters: a path with no reserved characters encodes to itself, because percent_encode_path leaves / alone. Presigned URLs for ordinary object names are byte-identical to what they are today; only names that are currently broken change.

What changes are included in this PR?

One call to percent_encode_path at each of the three sites, making presign agree with download_file_by_name and upload_file.

Tests

No new tests, deliberately: these are format! arguments and a header value inside an async method whose only seam is a live B2 authorization call, and asserting on them would mean restructuring the presign arms — which I would rather not fold into a fix. The 3 existing unit tests pass; cargo fmt --all -- --check and cargo clippy -p opendal-service-b2 --all-targets are clean with zero warnings.

Note for rebasing

#7801 renames build_abs_path at these same three sites. If it lands first, the X-Bz-File-Name line needs a one-word rebase.

Are there any user-facing changes?

Yes, for services-b2: presign_read / presign_stat / presign_write on an object whose name contains a character that is reserved in a URL now address that object, keep their authorization token, and agree with what a non-presigned read or write would do. Presigned URLs for names without reserved characters are unchanged.

[Xuanwo]

Next time, please avoid submitting similar PRs.