feat(dynamic): extended message schema

[schmidma]

Description

GitHub shows this PR against main, but the intended review base is #139 and #164.

This PR adds optional extended schema discovery for Rust-native message types that cannot be represented in the standard ROS 2 type description language, such as Option<T> and enums.

The existing MessageTypeInfo path remains strict and unchanged. ROS-compatible structs continue to use the standard ROS 2 schema service, while richer Rust shapes can use the separate extended schema mechanism between ros-z nodes.

Checklist

• Ran ./scripts/check-local.sh successfully
• Added or updated tests and documentation when applicable

[codecov]

Codecov Report

❌ Patch coverage is 63.88022% with 579 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/ros-z/src/extended_schema.rs	64.32%	178 Missing ⚠️
crates/ros-z/src/dynamic/serialization/cdr.rs	28.92%	86 Missing ⚠️
crates/ros-z/src/dynamic/value.rs	0.00%	63 Missing ⚠️
crates/ros-z/src/node.rs	68.86%	33 Missing ⚠️
crates/ros-z/src/service.rs	77.77%	32 Missing ⚠️
crates/ros-z/src/action/client.rs	11.76%	30 Missing ⚠️
crates/ros-z/src/graph.rs	72.89%	29 Missing ⚠️
crates/ros-z/src/dynamic/schema.rs	43.47%	26 Missing ⚠️
crates/ros-z/src/dynamic/discovery.rs	80.76%	25 Missing ⚠️
crates/ros-z/src/dynamic/type_description.rs	13.33%	13 Missing ⚠️
... and 9 more

Files with missing lines	Coverage Δ
crates/ros-z/src/action/server.rs	74.38% <ø> (ø)
crates/ros-z/src/config.rs	81.62% <ø> (ø)
crates/ros-z/src/context.rs	51.19% <100.00%> (+0.59%)	⬆️
...ates/ros-z/src/dynamic/type_description_service.rs	61.61% <100.00%> (+30.93%)	⬆️
crates/ros-z/src/encoding.rs	95.37% <ø> (ø)
crates/ros-z/src/event.rs	87.57% <100.00%> (+0.25%)	⬆️
crates/ros-z/src/lib.rs	100.00% <ø> (ø)
crates/ros-z/src/parameter/client.rs	98.38% <100.00%> (ø)
crates/ros-z/src/parameter/service.rs	78.74% <100.00%> (ø)
crates/ros-z/src/ros_msg.rs	48.64% <100.00%> (+4.53%)	⬆️
... and 20 more

... and 3 files with indirect coverage changes

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