docs: add OAuth response examples

[basit3407]

Split out from PR #167 so OAuth/OIDC response examples can be reviewed independently.

Scope:

• Adds token exchange examples for authorization-code and refresh-token flows.
• Adds token/introspection/userinfo error examples.
• Adds active and inactive introspection examples.
• Adds authorize endpoint invalid request/redirect URI examples.

Local validation:

• Parsed openAPI/oauth2-apis/v1.json with Node.
• Checked every OAuth JSON response has a top-level example.
• git diff --check

Hosted validation:

• Cloudflare Pages passed.

[cloudflare-workers-and-pages]

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[Copilot]

Pull request overview

This PR updates the OAuth2 OpenAPI specification to include concrete JSON response examples for key OAuth/OIDC endpoints, making the docs more useful for client implementers and easier to validate in tooling.

Changes:

• Added success examples for /oauth2/token covering authorization-code and refresh-token exchanges.
• Added error examples for /oauth2/token, /oauth2/introspect, /oauth2/auth, and /userinfo.
• Added active/inactive token examples for /oauth2/introspect.

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