Fix empty flags in Attributes::from_iter(AttributeValue)

[digizeph]

Summary

Fix attribute flag initialization when constructing Attributes from AttributeValue iterators.

Changes

• Update FromIterator for Attributes to map via Attribute::from.
• Remove manual construction that set AttrFlags::empty().
• Add regression test to assert ORIGIN and AS_PATH created through Attributes::from_iter(...) are marked transitive.
• Add an Unreleased changelog entry under Bug fixes.

Example

Before this change, Attributes::from_iter(vec![AttributeValue::Origin(Origin::IGP)]) encoded ORIGIN with flag byte 0x00.
After this change, it encodes with the correct transitive flag 0x40.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.10%. Comparing base (3060b00) to head (9371c2e).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #264   +/-   ##
=======================================
  Coverage   91.09%   91.10%           
=======================================
  Files          83       83           
  Lines       14974    14982    +8     
=======================================
+ Hits        13641    13649    +8     
  Misses       1333     1333           

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