feat(codec): add Zenoh message fuzz target

[evshary]

Description

This PR adds an initial cargo-fuzz setup for zenoh-codec and introduces a focused fuzz target for TransportMessage decoding.

What does this PR do?

The change adds a standalone fuzz crate under commons/zenoh-codec/fuzz, a raw-byte fuzz target for TransportMessage, deterministic seed corpus generation/verification helpers, and a lightweight CI smoke job to keep the fuzz target buildable and exercised

Why is this change needed?

• Adds a new cargo-fuzz crate at commons/zenoh-codec/fuzz.
• Implements a shared fuzz harness for TransportMessage decoding.
• Separates two fuzzing concerns in the harness:
  • decode robustness for arbitrary input
  • codec consistency via decode/encode/decode roundtrip
• Documents the fuzz workflow in commons/zenoh-codec/README.md.
• Adds a CI smoke job

Related Issues

#2592

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🏷️ Label-Based Checklist

Based on the labels applied to this PR, please complete these additional requirements:

Labels: enhancement

✨ Enhancement Requirements

Since this PR enhances existing functionality:

• Enhancement scope documented - Clear description of what is being improved
• Minimum necessary code - Implementation is as simple as possible, doesn't overcomplicate the system
• Backwards compatible - Existing code/APIs still work unchanged
• No new APIs added - Only improving existing functionality
• Tests updated - Existing tests pass, new test cases added if needed
• Performance improvement measured - If applicable, before/after metrics provided
• Documentation updated - Existing docs updated to reflect improvements
• User impact documented - How users benefit from this enhancement

Remember: Enhancements should not introduce new APIs or breaking changes.

Instructions:

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.99%. Comparing base (0aa7578) to head (e024036).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2591      +/-   ##
==========================================
- Coverage   74.02%   73.99%   -0.03%     
==========================================
  Files         400      400              
  Lines       60864    60864              
==========================================
- Hits        45056    45039      -17     
- Misses      15808    15825      +17     

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