feat(dynamic_message): add DynamicSequence length helpers

[bresilla]

What

Adds small ergonomic helpers to dynamic message sequences:

• len() / is_empty() for DynamicSequence, DynamicBoundedSequence, DynamicSequenceMut, DynamicBoundedSequenceMut
• iter() for immutable sequences

Why

Makes it easier to traverse dynamic message sequences (especially sequences of nested messages) without reaching into internal storage.

Notes

• No behavior change; purely additive API.
• cargo test passes.

[Copilot]

Pull request overview

This PR adds ergonomic helper methods to dynamic message sequence types to improve ease of use when working with nested messages.

Changes:

• Added len() and is_empty() methods to all four sequence types for querying sequence length
• Added iter() method to immutable sequence types (DynamicSequence and DynamicBoundedSequence)
• Reformatted imports to use nested import syntax

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[luca-della-vedova]

I believe but please correct me if I'm wrong that since all the types implement Deref which just returns the inner slice, we can just deref the sequences through * then we should be able to use all of Rust's slice APIs (i.e. is_empty) so we wouldn't need to implement any ourselves