fix(openapi): avoid OAuth scope type collisions

[ngyna]

Closes #17411. The OpenAPI importer now preserves a user-defined OAuthScope instead of letting Fern's synthetic OauthScope enum converge on the same generated file path.

• Defer the synthetic enum until all definitions are emitted, then deconflict its name against the actual root-package types.
• Keep OauthScope for unaffected APIs and choose a deterministic OauthAuthorizationScope fallback only on a genuine case-insensitive collision.
• Cover component, inline-generated, and webhook-response collisions while preserving existing grouped, unreferenced, and request-inline behavior.

[nitpickybot]

AI Review Summary

Adds collision-avoidance for the synthetic OauthScope enum in the OpenAPI importer, plus a fixture and snapshots. The core logic is straightforward, but the collision scan is both too coarse (ignores groupName, so it renames even when the colliding schema lands in a different file) and too narrow (ignores namespaced schemas), and getSchemaName can return undefined for schema variants without a generated name, which would blow up on .toLowerCase().

• 🟡 2 warning(s)
• 🔵 2 suggestion(s)

[devin-ai-integration]

Devin Review found 1 potential issue.

View 1 additional finding in Devin Review.