Fix gossip scouting for loopback locators

[fuzzypixelz]

Description

This patch fixes a regression introduced in #2671: loopback locators were no longer being sent during gossip scouting.

What does this PR do?

This patch restores the old scouting behavior where loopback locators were filtered out only if they originate in unspecified endpoints.

I've also caught an inadvertant change of behavior in SessionInfo::locators introduced in #2671, where said method started returning the aforementioned loopback locators originating in unspecified endpoints.

Related Issues

Fixes #2753.

---

🏷️ Label-Based Checklist

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

Labels: bug

🐛 Bug Fix Requirements

Since this PR is labeled as a bug fix, please ensure:

• Root cause documented - Explain what caused the bug in the PR description
• Reproduction test added - Test that fails on main branch without the fix
• Test passes with fix - The reproduction test passes with your changes
• Regression prevention - Test will catch if this bug reoccurs in the future
• Fix is minimal - Changes are focused only on fixing the bug
• Related bugs checked - Verified no similar bugs exist in related code

Why this matters: Bugs without tests often reoccur.

Instructions:

1. Check off items as you complete them (change - [ ] to - [x])
2. The PR checklist CI will verify these are completed

This checklist updates automatically when labels change, but preserves your checked boxes.

[codecov]

Codecov Report

❌ Patch coverage is 82.35294% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.70%. Comparing base (c5d4760) to head (fa4fe7e).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
io/zenoh-transport/src/unicast/manager.rs	70.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2755      +/-   ##
==========================================
- Coverage   74.70%   74.70%   -0.01%     
==========================================
  Files         419      419              
  Lines       63825    63820       -5     
==========================================
- Hits        47681    47675       -6     
- Misses      16144    16145       +1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.