fix(webdav): retry stale digest authentication

[otiscuilei]

What this changes

Refreshes Digest authentication when a WebDAV server rejects an expired nonce with a new WWW-Authenticate challenge containing stale=true.

Previously, the first Digest challenge was handled, but any later 401 from an already active DigestAuth immediately returned an authorization error. Servers that periodically expire nonces therefore became unusable until the client was recreated.

The retry is deliberately bounded to one stale challenge so a misbehaving server cannot cause unbounded recursion. Responses are closed before retrying so the underlying connection can be reused safely.

Tests

• Added an httptest WebDAV flow covering initial authentication, nonce expiry, refreshed authentication, and success
• Added a repeated-stale test proving the retry is bounded
• go test ./pkg/gowebdav -count=1
• All pkg/... tests pass except the existing pkg/aria2/rpc integration tests, which require an aria2 server on localhost:6800

Fixes #7556

[okatu-loli]

This looks solid to me. Checked the DigestAuth struct literal lines up with the field order {user, pw, digestParts}, the retry is bounded to a single stale challenge so theres no runaway recursion, and the response body gets closed before each retry. Tests cover both the happy path and the bounded case, which is what I wanted to see.

One tiny thing, not a blocker: the !canRetry stale branch returns without closing rs.Body, but the neighboring 401 branches do the same, so at least its consistent. Happy to merge.