Prevent concurrent OAuth2 client authorization requests

[yashdotdev13]

Summary

Prevent duplicate OAuth2 client authorization requests when multiple threads concurrently authorize the same client registration and principal.

Changes

• Coordinate in-flight authorization requests using OAuth2AuthorizedClientId and CompletableFuture.
• Ensure only one concurrent request invokes the OAuth2AuthorizedClientProvider for a given client registration and principal.
• Have concurrent callers wait for and reuse the in-flight authorization result.
• Propagate authorization failures to waiting callers and remove completed in-flight requests safely.
• Add a regression test covering concurrent authorization.

Validation

• ./gradlew :spring-security-oauth2-client:check
• ./gradlew :spring-security-oauth2-client:test
• ./gradlew :spring-security-oauth2-client:checkstyleMain :spring-security-oauth2-client:checkstyleTest

The branch was rebased onto the latest upstream/main before publishing.