fix(qbittorrent): support qBittorrent 5.2+ session cookies and JSON add responses

[pindaroli]

Summary

This PR fixes compatibility with qBittorrent v5.2.0 and later releases.

Background & Issues Fixed

1. Session Cookie Name Breaking Change:
   Starting with qBittorrent 5.2.0, WebUI session cookies are formatted as QBT_SID_<PORT> (e.g. QBT_SID_8080) rather than the legacy SID. The previous strict check login.cookies.get("SID") returns None and raises qBittorrent login failed: no SID cookie returned. even when credentials are valid and a session cookie is returned.

   • Fix: Dynamically inspect returned cookies for any cookie containing SID, falling back to SID for older versions.

2. Subnet Whitelist Bypass & HTTP 204 Handling:
   In qBittorrent 5.2.0+, /api/v2/auth/login returns HTTP 204 No Content on success, or when subnet whitelist bypass is enabled (WebUI\AuthSubnetWhitelistEnabled=true), it may not issue any cookie at all.

   • Fix: Handle 204 / subnet whitelist bypass scenarios gracefully without failing.

3. Structured JSON Responses in /api/v2/torrents/add:
   Modern qBittorrent endpoints return structured JSON dicts (e.g. {"added_torrent_ids": [...], "success_count": 1, "failure_count": 0}) rather than plain text "Ok.". Previously, _qbt_add_result only checked for isinstance(result, str) and "ok.", causing successful additions to be reported as ❌ Failed to add magnet.

   • Fix: Added support for dictionary returns in _qbt_add_result, checking success_count and added_torrent_ids.

Testing

• Tested against live qBittorrent v5.2.3 instances.
• Successfully verified authentication, torrent listing (qbt_list_torrents), torrent details (qbt_torrent_details), and magnet additions (qbt_add_magnet).