Reduces allocation and memory copies for processing JSON messages

[kahrendt]

Uses const char pointers directly into the websocket payload instead of creating an intermediate string copy that gets passed around.

[Copilot]

Pull request overview

This PR changes the JSON message delivery path to avoid allocating/copying an intermediate std::string by passing a (char* data, size_t len) view directly into the connection’s WebSocket reassembly buffer, and parsing from that buffer in-place.

Changes:

• Update on_json_message_cb (and related call sites) to take a pointer + length instead of std::string.
• Update SendspinConnection::dispatch_completed_message() to invoke the JSON callback directly on the reassembly buffer (no std::string construction).
• Update SendspinClient::process_json_message() to deserialize JSON from (data, len).

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/host/client_connection.h	Updates documentation example for the new JSON callback signature.
src/connection.h	Updates JSON callback type + documentation and describes new dispatch behavior.
src/connection.cpp	Removes std::string copy during dispatch; passes buffer pointer/len to callback.
src/connection_manager.cpp	Propagates new callback signature to client JSON processing.
src/client.cpp	Parses JSON directly from the connection buffer via (data, len).
include/sendspin/client.h	Updates private client JSON processing signature and docs to match new buffer-based parsing.

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