unit test: Add parallel encoding regression test

[hanguyen-nuro]

Summary

Adds a regression test to ensure processEncodedFrames processes tracks in parallel. This guards against future changes that might inadvertently serialize VP8 encoding.

How it works

• Injects 10 mock tracks, each with a 50ms encode delay in Read()
• Calls processEncodedFrames() and measures wall-clock time
• Asserts total time < 150ms (3x single encode for generous margin)
• Sequential execution would take ~500ms (10 × 50ms) and fail the assertion

Verified

• Passes with parallel processEncodedFrames (current master): ~50ms
• Fails with sequential version (pre-e94246f): ~501ms

=== RUN   TestRTCSender_ProcessEncodedFrames_Parallel
    Error: "501.915234ms" is not less than "150ms"
--- FAIL (sequential)

=== RUN   TestRTCSender_ProcessEncodedFrames_Parallel
--- PASS: 50ms (parallel)

Flakiness

• Ran 10x with -count=10: 10/10 pass
• 3x threshold (150ms vs 50ms actual) provides generous margin for CI variance

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 42.23%. Comparing base (e94246f) to head (6d97fbb).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #143   +/-   ##
=======================================
  Coverage   42.23%   42.23%           
=======================================
  Files          18       18           
  Lines        1636     1636           
=======================================
  Hits          691      691           
  Misses        886      886           
  Partials       59       59           

Flag	Coverage Δ
go	42.23% <ø> (ø)

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