feat: add close_link method to TransportUnicast API

[gabrik]

Summary

Add close_link(Link) to the TransportUnicastTrait and expose it through the TransportUnicast public wrapper. This allows closing a specific link within a multilink transport without tearing down the entire transport.

Changes

• TransportUnicastTrait: new async fn close_link(&self, link: Link) -> ZResult<()> on the trait
• TransportUnicastUniversal: delegates to existing del_link()
• TransportUnicastLowlatency: closes the entire transport (it has at most one link)
• MockTransportUnicastInner: no-op stub for tests
• TransportUnicast: public close_link() wrapper exposed with doc example

Motivation

This is a prerequisite for transport link rotation (see #2746), where the rotation engine needs to close individual links within a multilink transport during make-before-break cycles. The existing del_link method on TransportUnicastUniversal was pub(crate) and not exposed through the trait or the public wrapper.

Tests

• transport_unicast_close_link_multilink: closes one of two links, verifies transport survives, then closes the last link and verifies transport closes
• transport_unicast_close_link_single_link: closes the only link, verifies transport closes

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🏷️ Label-Based Checklist

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

Labels: new feature

🆕 New Feature Requirements

Since this PR adds a new feature:

• Feature scope documented - Clear description of what the feature does and why it's needed
• Minimum necessary code - Implementation is as simple as possible, doesn't overcomplicate the system
• New APIs well-designed - Public APIs are intuitive, consistent with existing APIs
• Comprehensive tests - All functionality is tested (happy path + edge cases + error cases)
• Examples provided - Usage examples in code comments or separate example files
• Documentation added - New docs explaining the feature, its use cases, and API
• Feature flag considered - Can the feature be enabled/disabled for gradual rollout?
• Performance impact assessed - Memory, CPU, storage implications measured
• Integration tested - Feature works with existing features

Consider: Can this feature be split into smaller, incremental PRs?

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[codecov]

Codecov Report

❌ Patch coverage is 44.44444% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.69%. Comparing base (8aaf85c) to head (ed8e049).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
io/zenoh-transport/src/unicast/mod.rs	33.33%	8 Missing ⚠️
io/zenoh-transport/src/unicast/test_helpers.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2745      +/-   ##
==========================================
+ Coverage   74.67%   74.69%   +0.01%     
==========================================
  Files         425      419       -6     
  Lines       63930    63837      -93     
==========================================
- Hits        47739    47680      -59     
+ Misses      16191    16157      -34     

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