Validate VN packet SourceCid

[gaurav2699]

Description

Resolves a long-standing TODO in QuicConnRecvVerNeg that noted the VN
packet's Source Connection ID should be validated against our current
Destination CID.

Testing

Do any existing tests cover this change? Are new tests needed?

Documentation

Is there any documentation impact for this change?

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.41%. Comparing base (1739655) to head (4f2d364).

Files with missing lines	Patch %	Lines
src/core/connection.c	71.42%	2 Missing ⚠️

❌ Your patch check has failed because the patch coverage (71.42%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6167      +/-   ##
==========================================
- Coverage   85.96%   85.41%   -0.56%     
==========================================
  Files          60       60              
  Lines       18974    18981       +7     
==========================================
- Hits        16311    16212      -99     
- Misses       2663     2769     +106     

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[anrossi]

You could see if updating QuicDrillTestServerVNPacket to test this is feasible