fix(services/cloudflare-kv): strip only the root prefix from listed keys

[PDGGK]

Which issue does this PR close?

None filed — reporting and fixing together.

Rationale of this change

CloudflareKvLister removed the service root from listed keys with str::replace:

let mut name = name.replace(root.trim_start_matches('/'), "");

str::replace removes every occurrence anywhere in the string, not the leading one. So any key that repeats the root as an inner path segment comes back mangled:

root	stored key	listed as	should be
/data/	data/backup/data/file.txt	backup/file.txt	backup/data/file.txt
/data/	xdata/file.txt	xfile.txt	xdata/file.txt

The second row shows it is not even anchored — a key merely containing the root loses that substring.

The same expression appeared at both call sites (build_entry_for_item for the key, and the non-recursive branch for self.path), so both now go through one relative_to_root helper.

On build_rel_path: that is what the s3/oss/cos/obs listers use here, and it was my first choice. It debug_assert!s that the path really does start with the root — a guarantee this service does not enforce on what the API returns — so a key that is not root-prefixed would panic in debug builds where replace previously left it alone. The helper therefore stays total and preserves that behaviour. Happy to switch to build_rel_path if you would rather have the assertion.

Are there any user-facing changes?

Yes: listed entry names are now correct for keys that repeat the root as an inner segment. Keys that do not are unaffected.

Tests

Three unit tests in lister.rs. Two fail against the previous expression:

test lister::tests::relative_to_root_does_not_match_mid_key ... FAILED
test lister::tests::relative_to_root_strips_only_the_prefix ... FAILED
test result: FAILED. 3 passed; 2 failed

The third covers the root itself and a bare / root and passes both before and after, which is what shows the other two are not trivially red.

With the fix: 5/5 pass (including the two pre-existing config tests). cargo fmt --check exits 0 and cargo clippy --all-targets is clean.