fix(auth): refine JSpecify nullability for ServiceAccountCredentials and UserCredentials

[lqiu96]

Description

Refines JSpecify nullability annotations and checks for ServiceAccountCredentials, UserCredentials, and their base hierarchy (GoogleCredentials, OAuth2Credentials, QuotaProjectIdProvider).

• JSpecify Type-Use Nullability: Annotates return types, fields, and builder methods with standard type-use @Nullable annotations.
• Guava Preconditions: Migrates from com.google.api.client.util.Preconditions to com.google.common.base.Preconditions.
• Null-Check Cleanup: Removes redundant null checks on constructor-validated non-null fields in UserCredentials.

[gemini-code-assist]

Code Review

This pull request updates several credential classes, including GoogleCredentials, OAuth2Credentials, QuotaProjectIdProvider, ServiceAccountCredentials, and UserCredentials, to adopt JSpecify nullability annotations. It also updates the OAuth2Credentials constructor to handle null margins by falling back to default values, and removes redundant null checks in UserCredentials. Feedback suggests explicitly annotating the refreshMargin and expirationMargin parameters in the OAuth2Credentials constructor with @nullable to align with the new null-handling logic.

[lqiu96]

/gemini review

[gemini-code-assist]

Code Review

This pull request improves null safety across several credential classes, including GoogleCredentials, OAuth2Credentials, QuotaProjectIdProvider, ServiceAccountCredentials, and UserCredentials, by systematically adding and standardizing @nullable annotations on fields, method parameters, and return types. It also migrates Preconditions imports from com.google.api.client.util to com.google.common.base and simplifies JSON serialization in UserCredentials by removing redundant null checks. No review comments were provided, so there is no additional feedback to address.