fix: make the test log handler coexist with rosout

[azerupi]

The test-only custom logging output handler (set_logging_output_handler, used by the rclrs logging tests) installed itself via rcutils_logging_set_output_handler, replacing the default rcl_logging_multiple_output_handler. While installed this had two process-global side effects:

1. /rosout (and console/file) publishing was disabled, because the default handler that fans out to them was no longer called. A test running concurrently that relied on /rosout (e.g. the rosout publishing test) could silently lose its messages.
2. Every log record emitted anywhere in the process was captured into the handler's buffer, so concurrent tests' logs polluted assertions such as last_message().

Under cargo's parallel test execution these made the logging tests intermittently fail.

Fix: the custom handler now

• chains to rcl_logging_multiple_output_handler, so console/file/rosout keep working while it is installed, and
• captures only records emitted on the thread that installed it (the test thread), so concurrently-running tests no longer pollute the captured output.

To keep the forwarded record printf-safe, impl_log now always calls rcutils_log with the "%s" + message form (instead of passing the message as the format string) and stashes the finished message in a thread-local that the handler reads back. This avoids parsing the C va_list in Rust while letting the record be forwarded to the default handler unchanged.

All changes are within #[cfg(test)] code, production logging is unchanged.

[maspe36]

This change seems fine in of itself,

breaking concurrently-running tests that rely on /rosout

but to me this is the real issue. Unit tests that depend on system level properties are actually integration tests and I argue shouldn't be run concurrently.

Do you have a list of these tests that would break concurrently?

[azerupi]

I believe it is only test_logging_macros because it replaces the logging out handler by calling set_logging_output_handler and this then intermittently breaks the other tests that subscribe to /rosout