C++: Add more thread creation models

[MathiasVP]

This is #19955, but for pthreads and std::threads.

Commit-by-commit review recommended.

[Copilot]

Pull Request Overview

This PR adds thread creation models for pthread and std::thread APIs to track data flow when arguments are passed to thread functions. The purpose is to improve CodeQL's ability to detect security vulnerabilities and data flow issues across thread boundaries.

• Adds data flow models for std::thread constructor to track parameter passing
• Adds data flow model for pthread_create function to track argument passing to thread functions
• Includes comprehensive test cases demonstrating taint flow through thread creation

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
thread.cpp	New test file demonstrating taint flow through std::thread creation
test.cpp	Added pthread_create test case showing taint flow to thread function
std.thread.model.yml	Data flow models for std::thread constructor parameter passing
pthread.model.yml	Data flow model for pthread_create argument passing
stl.h	Added std::thread class definition for testing
*.expected	Updated test expectations reflecting new thread models

[jketema]

Small nit, other than this LGTM if DCA is happy.