fix(ruby): qualify root module constants

[ngyna]

Closes #17413. Generated Ruby SDKs now load when a service group has the same module name as the SDK root: runtime references resolve from Ruby's top-level namespace instead of being redirected through the nested service.

• Make fully qualified Ruby AST references emit a leading ::, and use them for generated model, client, HTTP, error, environment, and request references.
• Qualify the runtime templates and small set of generator strings that bypass the AST.
• Add a generic Seed::Seed regression fixture that loads the gem and exercises root and nested clients, models, and enums.
• Regenerate the Ruby v2 seed corpus with only the resulting root-qualification changes; unrelated pre-existing seed drift is excluded.

[nitpickybot]

Reviewed the changes — everything looks good. No issues found.

[devin-ai-integration]

Devin Review found 1 potential issue.

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