Ignore malformed nomination attributes

[JoTurk]

Description

pion/ice had this bug for a while where it enter controlled nomination handling when we receive a malformed nomination-only binding request even without use-candidate. This isn't a big issue because we used connectivity checks to promote waiting candidates, but after #909 we don't do checks for LITE and we promote them directly to Succeeded, and this can cause problems with non standard ICE clients or clients that send malformed nominations.
I found this by looking at the selection code while reviewing and fixing #909 and it deserved its own PR / commit because it wasn't directly related.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.56%. Comparing base (4d201a4) to head (bcd6915).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #939      +/-   ##
==========================================
+ Coverage   88.35%   88.56%   +0.20%     
==========================================
  Files          46       46              
  Lines        5978     5982       +4     
==========================================
+ Hits         5282     5298      +16     
+ Misses        481      470      -11     
+ Partials      215      214       -1     

Flag	Coverage Δ
go	88.56% <100.00%> (+0.20%)	⬆️

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[Copilot]

Pull request overview

This PR fixes a long-standing issue where the controlled agent would treat presence of the nomination attribute as a nomination even when that attribute was malformed (and even without USE-CANDIDATE). This matters more after #909 because lite controlled agents can now directly promote pairs to Succeeded on nomination, so malformed nomination-only requests could incorrectly select a pair.

Changes:

• Only treat the custom nomination attribute as a nomination trigger when it parses successfully (valid length/value extraction).
• Keep USE-CANDIDATE behavior unchanged, even if a malformed nomination attribute is also present.
• Add regression tests covering malformed nomination-only requests vs USE-CANDIDATE + malformed nomination.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
selection.go	Requires a validly parsed nomination attribute before entering nomination handling; continues to accept USE-CANDIDATE nominations.
selection_test.go	Adds lite-mode regression tests ensuring malformed nomination-only requests do not nominate/select, while USE-CANDIDATE still does.

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[JoTurk]

@kevmo314 thank you for the fast review :)