`pactffi_get_plugin_logs` requires a `plugin_instance_id` that no FFI function exposes

[JP-Ellis]

Affects: pact-ffi 0.5.5 (introduced by #537)

Summary

const char *pactffi_get_plugin_logs(const char *plugin_instance_id);

There is no way for an FFI consumer to obtain a plugin_instance_id. Searching the full 0.5.5
pact.h (280 exported functions), the only occurrences of "instance_id" are in this function's
own signature and doc comment. pactffi_using_plugin / pactffi_using_plugin_with_delay return
only a c_uint status code, not an instance ID.

Internally the buffer is a HashMap keyed by entry.plugin_instance_id (from the plugin
driver), which is never surfaced across the FFI boundary.

Interaction with Issue 1

The only place a plugin_instance_id is ever exposed to a C caller is the first parameter of
the log callback registered via pactffi_register_plugin_log_callback. The intended flow
appears to be:

1. register the callback,
2. learn instance IDs as entries arrive,
3. call pactffi_get_plugin_logs for a given instance.

Since Issue 1 makes step 1 impossible, the entire plugin-observability surface added in #537 is
unreachable from a C consumer. Fixing Issue 1 unblocks this one.

Suggested fix

Fix Issue 1 (which makes this reachable), and/or add an accessor that returns the plugin instance
ID — for example an out-parameter on pactffi_using_plugin — so that pactffi_get_plugin_logs
is independently usable without registering a callback.