sdp: order simulcast rids by the a=simulcast attribute, not a=rid line order

[StefanoD]

Description

getRids returned the simulcast rids in a=rid line order, consulting a=simulcast only to mark ~ paused streams. Per RFC 8853 §5.2, the a=simulcast send list — not the a=rid line order — carries the stream preference:

The order of the listed simulcast streams in the "send" direction suggests a proposed order of preference, in decreasing order

When a peer lists its a=rid lines in a different order than its a=simulcast:send list, the resulting RTPReceiveParameters.Encodings / Tracks() order — and the a=simulcast:recv list echoed in the answer — followed the a=rid order instead of the offered preference.

This stable-sorts the rids by their first position in the a=simulcast attribute; any rid absent from it keeps its a=rid declaration order at the end.

Latent in practice — browsers (Firefox 152, Chrome) emit both in the same order, and demux is by RID string — so this is a SHOULD-level alignment, not a spec violation: §5.2 "suggests" the order (non-normative), and §5.3.2 only mandates reversing send↔recv (SHALL) and forbids adding streams (MUST NOT); it does not itself require preserving order.

Closes #3471 (full reproduction and impact analysis there).

Test plan

• Adds TestGetRIDsOrderFollowsSimulcast: an offer whose a=rid lines are in the opposite order (q, h, f) to a=simulcast:send f;h;q. getRids returns [q, h, f] before this change and [f, h, q] after.
• go build ./..., go vet ., gofmt, and golangci-lint run (v2.10.1, --new-from-rev=main) are all clean.

[JoTurk]

Thank you, once you fix the parser issue we can merge this.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.03%. Comparing base (d9b2a24) to head (145e1e8).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3472      +/-   ##
==========================================
- Coverage   86.18%   86.03%   -0.15%     
==========================================
  Files          81       81              
  Lines       10442    10456      +14     
==========================================
- Hits         8999     8996       -3     
- Misses       1007     1021      +14     
- Partials      436      439       +3     

Flag	Coverage Δ
go	86.03% <100.00%> (-0.15%)	⬇️

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