Add test case to exploit crashing bug in QuicSendFlush

[saikat107]

Description

This pull request a new unit test for the send logic. The main change is addition of a unit test for the send path, specifically targeting pacing logic, that potentially exposes a crashing bug in Quic Send logic.

Testing

Testing Enhancements:

• Added SendTest.cpp to the unit test build (src/core/unittest/CMakeLists.txt) and implemented a new test file for the send logic:
  • Introduced a mock context and a detailed test (FlushPacingDelayed) that validates the pacing behavior in the send path, including setup for connection state, congestion control, and timer logic. [1] [2]

To run the test

.\scripts\test.ps1 -Filter "*DeepTest_SendTest*" ...

The test will fail.

Documentation

N/A

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.80%. Comparing base (26c3dfa) to head (d8c57e6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5825      +/-   ##
==========================================
- Coverage   86.11%   84.80%   -1.31%     
==========================================
  Files          60       60              
  Lines       18729    18729              
==========================================
- Hits        16129    15884     -245     
- Misses       2600     2845     +245     

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