Replace the duplicated `OAUTH_RESULT_COOKIE` constant and inline payload
type across the callback route and connector dialog hook with a shared
`contracts/types/oauth.types.ts` module that exports:
- OAUTH_RESULT_COOKIE constant
- oauthCallbackResultSchema Zod schema
- OAuthCallbackResult type (inferred from the schema)
- parseOAuthCallbackResult() helper that returns null on invalid JSON
or shape mismatch
The route handler now uses the shared type to constrain the cookie
payload at compile time. The consumer hook validates the cookie value
through the helper instead of an unchecked JSON.parse, removing the
silent runtime risk when the cookie is tampered with or its shape
drifts.