feat: add plugin observability module

[JP-Ellis]

Goal

Pact FFI 0.5.5 added plugin observability: a per-thread test run ID sent to plugins, a log callback for entries from any running plugin, and a per-instance log buffer. The pact-python-ffi 0.5.8 release wrapped these; this PR surfaces them in the main library as pact.plugins.

What it adds

• set_test_run_id — tags plugin requests from the current thread so log entries can be attributed to a test.
• register_log_callback — delivers each entry as a PluginLogEntry dataclass.
• forward_to_logging — ready-made callback emitting through the standard logging module at the equivalent level, with plugin_instance_id, test_run_id and plugin_target attached to each record.
• get_logs — the entries buffered for a plugin instance, with timestamp_ms converted to a UTC datetime.

A single CFFI trampoline is registered with the library and dispatches to whichever Python callable is current. Upstream ignores a NULL registration, so this is what makes register_log_callback(None) actually stop delivery.

Prerequisite worth knowing

The FFI only installs its plugin log sink from pactffi_init / pactffi_init_with_log_level, not from log_to_stderr. Without one of those, the callback is silently never invoked. The module docstring and docs/logging.md direct users to pact_ffi.init_with_log_level in its place; ideally upstream would install the sink from pactffi_register_plugin_log_callback itself.

Verification

• 18 unit tests drive the registered C callback directly and cover level mapping, None normalisation, callback replacement and deregistration, and logging forwarding.
• End-to-end against a real plugin process: the bundled csv/protobuf plugins write nothing to stderr and predate the Log RPC, so a scratch plugin under PACT_PLUGIN_DIR that writes to stderr was used. Entries arrived through forward_to_logging with the extras populated, and get_logs returned the same entries with plugin_name, source="Stderr" and timestamps.
• mkdocs build resolves every cross-reference in the new module and the logging page.