chore: add parameter request issue template

[guillaumegay13]

Summary

• add a GitHub issue form for model parameter coverage requests
• collect provider, model, auth type, official docs, requested parameter details, and MPS scope confirmation
• leave the GitHub issue title empty so users must enter a concrete title, with a suggested <provider>/<model>: parameter coverage format

Validation

• Ruby YAML parse for .github/ISSUE_TEMPLATE/parameter-request.yml
• git diff --check

[vercel]

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