C#: Add models as data tests for compound assignment operators.

[michaelnebel]

No description provided.

[Copilot]

Pull request overview

Adds C# external-model test coverage for compound assignment operator overloads by introducing summary models for += (including checked context) and exercising them in the existing external-models dataflow test.

Changes:

• Added external summary models for op_AdditionAssignment and op_CheckedAdditionAssignment on My.Qltest.N.
• Added a new My.Qltest.N test type using operator += / operator checked += plus call sites that should produce taint flow.
• Regenerated the corresponding .expected output to include the new models and flow paths.

Show a summary per file

File	Description
csharp/ql/test/library-tests/dataflow/external-models/ExternalFlow.ext.yml	Adds two new summary model rows for compound assignment operator overloads (+= and checked +=).
csharp/ql/test/library-tests/dataflow/external-models/ExternalFlow.expected	Updates expected model list and provenance-path graph output to reflect the new operator tests.
csharp/ql/test/library-tests/dataflow/external-models/ExternalFlow.cs	Introduces N with compound assignment operators and two test methods that drive the modeled taint flow into Sink.

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 0