test: expand plan generation and replan regression fixtures

[chenrui333]

Summary

• add built-in and run-only custom plan-generation replan fixtures
• add a negative mixed workflow that mutates Atlantis's managed plan before built-in apply
• validate fixture directories, project/workflow references, documentation status, markers, and optional upstream E2E case references
• document which regression lifecycles are active versus expected-failure coverage

References

• fix: preserve custom apply workflows and autoplan state atlantis#6657
• Regression: Autoplan -> Apply fails atlantis#6641
• Regression (v0.46.0): atlantis apply fails with "plan file is missing" for custom workflows atlantis#6642
• follows test: add v0.46.0 custom plan-path apply regressions #21611

The new fixtures are activated by the coordinated E2E runner changes in runatlantis/atlantis#6657.