Add fuzz test for UnmarshalCandidate

[vnykmshr]

The other pion repos (stun, dtls, sdp) have fuzz tests for their parse paths but ice doesn't. UnmarshalCandidate handles remote input so it's a good candidate for coverage.

Adds FuzzUnmarshalCandidate with 14 seed corpus entries covering all candidate types, address families, TCP types, extensions, and edge cases (empty foundation, boundary ports, candidate: prefix). Asserts that successful parses produce candidates that marshal and re-parse without panicking.

1.6M+ executions locally, no crashes found.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.64%. Comparing base (883b73c) to head (c84deae).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #911      +/-   ##
==========================================
+ Coverage   88.60%   88.64%   +0.03%     
==========================================
  Files          45       45              
  Lines        5811     5811              
==========================================
+ Hits         5149     5151       +2     
+ Misses        453      451       -2     
  Partials      209      209              

Flag	Coverage Δ
go	88.64% <ø> (+0.03%)	⬆️

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[JoTurk]

Thank you. Can you move this to pre existing file? We try to limit our files count.

[theodorsm]

This is awesome @vnykmshr, love so see fuzzing being used across our repos!

One suggestion is to also search for uses of "MUST NOT" in the RFC to find invalid state for Candiate. You can use it to find assertions that are useful in fuzzing, like we do here in dtls: https://github.com/pion/dtls/blob/fd27a52a64265097a621432090926a651109b4cb/pkg/protocol/extension/certificate_auth_test.go#L137

But don't feel obliged to do more stuff in this PR, this is really useful already.

[JoTurk]

@theodorsm unlike dtls ouu ice parsers are a lot forgiving and we don't enforce the rfc in many cases like for example we allow for empty foundation, some invalid characters, bad addresses, and we also explicitly error in some gray areas in the rfc. I don't think we should enforce any parse behavior with fuzzing in ice.

[JoTurk]

Thank you :)