Lazy-import oauthlib/httpx and relax token type hint

[FeodorFitsner]

Move imports of oauthlib.WebApplicationClient and httpx from module top-level into the functions that use them to enable lazy importing and avoid import-time dependency issues. Also remove the explicit OAuth2Token type annotation from __convert_token to avoid importing oauthlib types at module import. These changes reduce startup import overhead and make the module safer when oauthlib/httpx may not be available until runtime.

Fix #6258

Summary by Sourcery

Make OAuth-related imports lazy and relax token typing to avoid import-time dependencies and reduce startup overhead.

Enhancements:

• Move oauthlib WebApplicationClient and httpx imports inside the authorization and token-handling methods to enable lazy loading.
• Remove the explicit OAuth2Token type annotation from the internal token conversion helper to prevent importing oauthlib types at module import time.

[sourcery-ai]

We've reviewed this pull request using the Sourcery rules engine

[Copilot]

Pull request overview

This PR introduces lazy imports for oauthlib and httpx within the authorization service to avoid import-time dependency issues and reduce startup overhead.

Changes:

• Moved httpx and oauthlib.oauth2.WebApplicationClient imports from module scope into the methods that use them
• Removed the OAuth2Token type annotation from __convert_token to avoid importing oauthlib token types at module import time

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[Copilot]

Removing the parameter type entirely loses useful static guarantees for callers and maintainers. To keep lazy-import behavior without depending on oauthlib at import time, consider using a stdlib type such as Mapping[str, object] / dict[str, object] (or Mapping[str, Any]) for t, or a local TYPE_CHECKING import to preserve the precise oauthlib type only for type checkers.

[ndonkoHenri]

lgtm