test: centralize test helpers and remove fixed ports from Zenoh test suites

[evshary]

Description

This PR introduces a shared zenoh-test crate and uses it to clean up test setup across zenoh, zenoh-ext, and io/zenoh-transport. The main goal is to remove hardcoded ports and reduce port-collision failures during parallel test execution.

What does this PR do?

• Adds a new workspace crate: commons/zenoh-test
  • Extracts reusable test utilities from zenoh/tests/common/mod.rs
  • Documents the helper patterns in commons/zenoh-test/README.md
• Refactors existing tests to use the shared helpers instead of ad hoc local utilities
• Renames the TCP helper to make its scope explicit
  • get_free_port() -> get_free_tcp_port()
  • adds get_free_udp_port() for UDP-based transport tests
• Updates crate-local zenoh tests to avoid fixed ports
  • zenoh/src/tests/session.rs
  • zenoh/src/tests/link_weights.rs
  • zenoh/src/tests/interceptor_cache.rs
• Updates zenoh-ext tests to use dynamic port allocation and the new helper crate
  • including advanced.rs and liveliness.rs
• Updates io/zenoh-transport tests to replace fixed TCP/UDP/WS/TLS/QUIC ports with dynamic allocation
  • includes endpoints.rs
  • includes the remaining unicast/multicast transport tests under io/zenoh-transport/tests

Related Issues

#2526

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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.

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[codecov]

Codecov Report

❌ Patch coverage is 89.65517% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.75%. Comparing base (d12952d) to head (85f3b63).
⚠️ Report is 3 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
commons/zenoh-test/src/lib.rs	89.65%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2573      +/-   ##
==========================================
+ Coverage   74.70%   74.75%   +0.04%     
==========================================
  Files         399      400       +1     
  Lines       59535    59716     +181     
==========================================
+ Hits        44478    44640     +162     
- Misses      15057    15076      +19     

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