PowerShell: fix enum string starting with a number

[wing328]

to close #21769

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Summary by cubic

Generate valid PowerShell enum member names when values start with digits by treating any leading-digit value as numeric and prefixing names (e.g., NUMBER_0).
Update the 3.0 Petstore spec and regenerate samples to add OuterEnumStringStartingWithNumber ('0', '1', '2'), with docs and a stub test.

^{Written for commit 318e0c9. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 7 files