chore: upgrade Workers OAuth provider to 0.9.0

[mattzcarey]

Summary

• upgrade the shared @cloudflare/workers-oauth-provider catalog entry from 0.8.2 to 0.9.0
• refresh the pnpm lockfile and add patch changesets for the direct consumers; Changesets propagates patch releases to every dependent Worker
• migrate the AI Gateway authentication integration fixture from the implicit flow to a real S256 authorization-code exchange, matching the stricter OAuth 2.1 validation in 0.9.0

Validation

• pnpm check:deps
• pnpm check:turbo
• pnpm check:format
• pnpm test (303 tests)
• pnpm turbo deploy -- -e staging --dry-run (18 Workers)
• pnpm changeset status
• git diff --check