refactor(core): move remote request handling into core behind remote-adapter feature

[LiamCarPer]

Part of #727.

Motivation

The redis, postgres and mongodb adapters each implement the same request handling: a near identical recv_req dispatch plus the same recv_* handlers (broadcast, broadcast_with_ack, rooms, add/del sockets, fetch sockets, disconnect sockets), and the same get_res skeleton (channel setup, response filter, take/timeout).

Solution

Added a RemoteRequestHandler trait in socketioxide-core behind the existing remote-adapter feature flag, with default implementations of the dispatch and of all the handlers. Each adapter only implements:

• local(), to access its CoreLocalAdapter
• send_res(req_id, req_origin, res), to send responses
• optionally recv_heartbeat(), a no-op by default. Postgres and mongodb delegate to the HeartbeatSender logic from refactor(core): move heartbeat/liveness logic into core behind remote-adapter feature #768.

Redis and mongodb keep a tiny wrapper that decodes the raw driver item and delegates, postgres calls the dispatch directly from its notification pipeline.

Also added insert_response_handler and wait_responses helpers covering the identical get_res channel setup, response type filter, take/timeout and handler cleanup.

Some tiny differences were normalized on purpose:

• send_res now takes (req_id, req_origin) everywhere with the strictest bounds (T: Serialize + Debug + Send + 'static, future + Send + 'static), redis's inverted order is handled inside its impl
• a request missing its options now warns and is skipped for all adapters (redis already did it, postgres and mongodb silently dropped it)
• the loopback check is done in the shared dispatch
• logs use the same style

The postgres get_res now also wraps the response stream in a DropStream, fixing a leak of response handler entries for rooms and fetch_sockets requests.

No behavior or wire-format change.

Tests

The existing adapter tests (fixture based, no real DB needed) cover every moved handler for the three adapters. Added 6 unit tests in core for the default dispatch: add/del sockets, heartbeat hook, loopback ignore, missing options, and the rooms / broadcast with ack responses.

[codspeed-hq]

Merging this PR will not alter performance

✅ 87 untouched benchmarks

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_{Comparing LiamCarPer:refactor/move-remote-request-handling-to-core (c8569d3) with main (9a10442)}