Transmission 4.1/4.2 API parity + JSON-RPC 2.0 support

[Copilot]

Transmission 4.1.0 deprecated the bespoke RPC protocol in favour of JSON-RPC 2.0 (all field names → snake_case, params/result instead of arguments, id instead of tag). Several new fields from 4.1.0 and 4.2.0 were also missing from the SDK.

Transparent JSON-RPC 2.0 protocol support

_FieldDict + _to_snake (types.py)

• _to_snake() converts camelCase/kebab-case → snake_case (compiled regex, e.g. hashString → hash_string, file-count → file_count)
• _FieldDict(dict) falls back to the snake_case equivalent key via __missing__ / __contains__ / get when a key isn't found directly
• Container.fields is now a _FieldDict — all existing property accessors work unchanged with both old camelCase/kebab responses and new JSON-RPC 2.0 snake_case responses

Auto-switching client (client.py)

• _http_query now accepts method + arguments and builds the wire query per retry iteration; this lets a protocol switch detected from the CSRF 409 X-Transmission-Rpc-Version header take effect on the immediate retry — no extra round-trip
• When rpc-version >= 18: emits {"jsonrpc":"2.0","method":"torrent_get","params":{…}} with all keys/field-list entries converted to snake_case
• Response parsing handles both formats: JSON-RPC 2.0 (error / result) and legacy (result:"success" / arguments); res is wrapped in _FieldDict so torrent_added vs torrent-added differences resolve automatically
• _update_server_version sets __use_jsonrpc2 = protocol_version >= 18

# SDK auto-detects; no API changes needed for callers
client = Client()  # connects to Transmission 4.1.0+
# Uses JSON-RPC 2.0 automatically; property accessors unchanged:
torrent.hash_string   # works whether server returned hashString or hash_string
session.rpc_version   # works whether server returned rpc-version or rpc_version

New fields (Transmission 4.1.0 / rpc-version 18)

Area	New fields
Torrent	sequential_download_from_piece
TrackerStats	downloader_count
Tracker	sitename
Session	preferred_transports, sequential_download, cache_size_mib, anti_brute_force_enabled, reqq, session_id
set_session()	preferred_transports, sequential_download, cache_size_mib; encryption now accepts "allowed"
add_torrent() / change_torrent()	sequential_download_from_piece

New fields (Transmission 4.2.0 / rpc-version 19)

• Torrent.webseeds_ex → list of new WebseedEx objects (url, is_downloading, download_bytes_per_second); exported from the top-level package

Original prompt

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This section details on the original issue you should resolve

<issue_title>4.1.0 Readiness?</issue_title>
<issue_description>Heya! I'm a downstream user of this package and I was just curious if anyone's checked against what is currently missing from the updated API. Would be nice if there's a list/tool to programatically check what needs to be done to bring this package to parity.</issue_description>

<agent_instructions>check changelog of transmission to see if our sdk work with latest version of transmission </agent_instructions>

Comments on the Issue (you are @copilot in this section)

@trim21 I guess no one. I haven't check changes of transmission yet

• Fixes 4.1.0 Readiness? #599

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