get_definitions() already wrapped check_fn() calls in try/except,
but is_toolset_available() did not. A failing check (network error,
missing import, bad config) would propagate uncaught and crash the
CLI banner, agent startup, and tools-info display.
Now is_toolset_available() catches all exceptions and returns False,
matching the existing pattern in get_definitions().
Added 4 tests covering exception handling in is_toolset_available(),
check_toolset_requirements(), get_definitions(), and
check_tool_availability().
Closes#402
logger.error() only records the exception message string, silently
discarding the stack trace. Switch to logger.exception() which
automatically appends the full traceback to the log output.
Without this change, when a tool handler raises an unexpected error
the log shows only the exception type and message, making it
impossible to determine which line caused the failure or trace
through nested calls.
