fix: preserve configured reqwest client

[DaleSeo]

Motivation and Context

Following up on PR #908, this fixes a regression from the OAuth HTTP client adapter change where AuthorizationManager::with_client no longer preserved the caller-provided reqwest configuration for token operations. Existing users of with_client(reqwest::Client) expect their proxy, TLS, connector, timeout, and default-header settings to apply across OAuth requests, including authorization-code exchange.

The regression happened because the adapter kept the configured reqwest client for redirect-following requests, but built a separate default client for OAuthHttpRedirectPolicy::Stop. That preserved the new no-redirect intent for token requests, but it also dropped any caller-provided reqwest configuration on those requests.

This fix preserves the legacy with_client contract by using the configured reqwest client for both redirect policies on that path, while leaving the newer OAuthHttpClient path available for callers that need stricter redirect handling.

How Has This Been Tested?

Added regression tests

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed