```mcap-ros2-support``` Fails to Serialize ```float64[]``` Array Fields (NumPy Conflict)

[hungngyenn]

Description

When attempting to serialize a ROS 2 message (like sensor_msgs/msg/Imu) where a fixed-size array field (e.g., orientation_covariance: float64[9]) is initialized as a NumPy ndarray, the mcap-ros2-support library throws a ValueError. The serialization checker strictly expects a standard Python sequence (list or tuple) for array types, even though the array is numeric.

• Version:
  mcap-ros2-support (PyPI)
  mcap 1.3.0 (PyPI)

• Platform:
  ROS2 Jazzy
  Python 3.12

Steps To Reproduce

1. Set up a minimal ROS 2 package environment with the recorder running.
2. Publish a sensor_msgs/msg/Imu message using a tool (e.g., ros2 topic pub) that initializes the fixed-size array fields (like orientation_covariance) as a NumPy ndarray object.

Note: The issue persists even when initializing with a Python list structure, indicating the conversion to ndarray happens upstream of the writer.

Expected Behavior
The message should be successfully serialized and recorded, as the fixed-size array conversion should be handled by the installed mcap-ros2-support version.

** Actual Behavior**

ValueError: Field "orientation_covariance" is not an array (<class 'numpy.ndarray'>) but has array type "float64[]"
...
File "/home/user/.local/lib/python3.12/site-packages/mcap_ros2/_dynamic.py", line 431, in _write_complex_type
    raise ValueError( 

Proposed Fix Location
The fix likely needs to ensure that numpy.ndarray objects for numeric fixed-size arrays (float64[]) are explicitly handled within the dynamic encoding logic:

mcap/python/mcap-ros2-support/mcap_ros2/_dynamic.py

Line 427 in 8810067

if (

[linear]

FG-13283

[james-rms]

I consider this a feature request. We could certainly set up the serialization to take compatible numpy arrays, but we would most likely need to take a dependency on numpy within mcap-ros2-support to do that, which I don't love. We could avoid taking the dependency by depending on a duck-type of the np array API.