Increase async cache stale period from 3 to 5 minutes. (#608)

## Changes

Increases the default async cache stale period from 3 minutes to 5
minutes.

## Rationale

The 5-minute stale period covers the maximum monthly downtime allowed by
a 99.99% uptime SLA (~4.38 minutes). This ensures that even during the
worst-case downtime scenario, the cache can continue serving stale
tokens while waiting for the auth service to recover (assuming
continuous traffic).

Co-authored-by: Claude Sonnet 4.5 <[email protected]>

Author: renaudhartert-db

NEXT_CHANGELOG.md

@@ -4,6 +4,8 @@
 
 ### New Features and Improvements
 
+* Increase async cache stale period from 3 to 5 minutes to cover the maximum monthly downtime of a 99.99% uptime SLA.
+
 ### Bug Fixes
 
 ### Security Vulnerabilities

databricks-sdk-java/src/main/java/com/databricks/sdk/core/oauth/CachedTokenSource.java

@@ -30,8 +30,9 @@ private enum TokenState {
   }
 
   private static final Logger logger = LoggerFactory.getLogger(CachedTokenSource.class);
-  // Default duration before expiry to consider a token as 'stale'.
-  private static final Duration DEFAULT_STALE_DURATION = Duration.ofMinutes(3);
+  // Default duration before expiry to consider a token as 'stale'. This value is chosen to cover
+  // the maximum monthly downtime allowed by a 99.99% uptime SLA (~4.38 minutes).
+  private static final Duration DEFAULT_STALE_DURATION = Duration.ofMinutes(5);
   // Default additional buffer before expiry to consider a token as expired.
   // This is 40 seconds by default since Azure Databricks rejects tokens that are within 30 seconds
   // of expiry.