[diffusion] chore: set allowing overriding protected fields of sampling params as default behavior

[mickqian]

And:

1. some very small refactors
2. testcase resuming

Motivation

Modifications

Accuracy Tests

Benchmarking and Profiling

Checklist

• Format your code according to the Format code with pre-commit.
• Add unit tests according to the Run and add unit tests.
• Update documentation according to Write documentations.
• Provide accuracy and speed benchmark results according to Test the accuracy and Benchmark the speed.
• Follow the SGLang code style guidance.
• Work with maintainers to merge your PR. See the PR Merge Process

[gemini-code-assist]

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[mickqian]

/tag-and-rerun-ci