Refactor Buildkite and Github CA, fix JET reports

[Krastanov-agent]

Summary

• Simplified the Buildkite pipeline configuration by using the matrix feature instead of duplicating step definitions
• Follows the cleaner pattern used in QuantumClifford.jl for consistency across the ecosystem
• Maintains all existing test configurations (base tests, JET, CUDA, OpenCL) and custom commands
• Refactored downstream tests to use matrix configuration for better maintainability
• Reduces code duplication and makes the pipeline easier to maintain

Test plan

• Verify all existing test jobs still run correctly
• Confirm matrix configuration properly sets environment variables
• Ensure custom Julia development commands continue to work
• Validate downstream QuantumOptics tests still execute properly
• Ensure test coverage and reporting continue to work as expected

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