fix(auth): settle token refresh launch failures

[sylvesterkaczmarek]

Fixes #425.

Catch provider exceptions raised before getAsync returns its future and route them through completeAsync. This releases the in-flight slot, settles the same waiting futures and applies advisory backoff consistently with asynchronous failures.

Reuse the protected launch path for forced refreshes chained after earlier requests. The chained provider launch uses whenCompleteAsync with an explicit common-pool executor, following the repository callback rule rather than executing on the previous refresh's completing thread.

Validation

Five new tests cover foreground/mandatory launch failures, advisory stale-token delivery and backoff with an injected clock, joined forced-refresh callers, recovery and an exceptional-future control.

• Unchanged main: four failures and one passing control.
• Auth, credential-provider and credential-cache suites: 87 tests passed, zero skips.
• Kotlin formatting and the complete core Kotlin format check passed.
• Type-resolved Detekt checks passed on the two changed files with existing rules and baselines. The initial full-core Detekt attempt exhausted its configured heap and was cancelled; a full repository lint pass is not claimed.
• Whitespace checks passed.

./gradlew :anthropic-java-core:test --tests 'com.anthropic.auth.*' --tests 'com.anthropic.credentials.*' --tests 'com.anthropic.config.CredentialsCacheTest'

Tested on macOS arm64 with JDK 21.0.12.1. Live authentication, full API tests and Java 8 runtime execution were not run. No public signatures or dependencies changed. Launch failures now produce exceptionally completed futures instead of escaping synchronously and leaving later requests pending.

[sylvesterkaczmarek]

The auth and credential suites pass (87 tests), together with the changed-file Detekt checks and core Kotlin formatting check. Could a maintainer approve the pending CI workflow?

[tomer-ant]

Closing along with #425. The reasoning is in #425 (comment).