Exclude server_version from Synchronization mutex

[nvasilevski]

Problem

Trilogy#server_version is wrapped by the Synchronization mutex, but it is a pure memory read with no socket I/O. This causes SynchronizationError when instrumentation libraries call server_version from within another synchronized method.

In practice, OpenTelemetry instrumentation (private to our company) wraps Trilogy#query in a span and calls server_version inside the span block to record the server version as a span attribute. Since query is synchronized, calling server_version from within it re-enters the mutex and raises SynchronizationError:

query (synchronized — holds mutex)
  └─ OTel query patch
       └─ client_attributes
            └─ server_version (synchronized — raises SynchronizationError)

This is the same class of problem as #276, which was fixed for closed? in #277.

Why server_version is safe to exclude

server_version returns a fixed char[] copied from the MySQL handshake greeting packet at connect time. It is never modified after initialization and performs no socket I/O.

The C implementation:

// Pure struct read — no socket interaction
static VALUE rb_trilogy_server_version(VALUE self) {
    return rb_str_new_cstr(get_open_ctx(self)->server_version);
}

The field is written once during _connect and never again:

memcpy(ctx->server_version, handshake.server_version, TRILOGY_SERVER_VERSION_SIZE);
ctx->server_version[TRILOGY_SERVER_VERSION_SIZE] = 0;

This makes it identical in safety characteristics to closed?, which is already excluded

[composerinteralia]

Makes sense to me. Thank you