feat: add in-memory OAuth token cache for read-only containers

[maierthomas]

Description

Adds an in-memory OAuthCredentialsCache implementation that can be activated via the ZEEBE_CLIENT_DISK_CACHE_DISABLE environment variable. When set, the client uses an in-memory token cache instead of persisting credentials to ~/.camunda/credentials.

Additional context

When running Zeebe clients in containers with read-only filesystems (readOnlyRootFilesystem: true in Kubernetes), the default YAML-based credentials cache fails because it cannot create or write to the cache file.
While a custom OAuthCredentialsCache can be injected programmatically via OAuthProviderConfig.Cache, an environment variable toggle is simpler for container deployments.

Note: If a custom cache is explicitly passed via OAuthProviderConfig.Cache, the environment variable is ignored.

Testing changes

• 3 new unit tests for the in-memory cache (TestInMemoryCacheGetAndUpdate, TestInMemoryCacheRefreshIsNoOp, TestInMemoryCacheGetMiss)
• 2 new integration tests for env var wiring (TestInMemoryCacheUsedWhenEnvSet, TestYamlCacheUsedWhenEnvNotSet)
• All existing tests pass: go test ./pkg/zbc/...
• go vet ./pkg/zbc/... clean

Types of changes

• Bug fix (non-breaking change which fixes an existing open issue)
• New feature (non-breaking change which adds functionality to an extension)
• Breaking change (fix or feature that would cause existing functionality of an extension to change)
• Documentation update (changes made to an existing piece of documentation)

Checklist:

• My code adheres to the syntax used by this extension.
• My pull request requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the Camunda Community Hub documentation.
• I have read the Pull Request Process documentation.
• I have added or suggested tests to cover my changes suggested in this pull request.
• All new and existing CI/CD tests passed.
• I will tag @/camunda-community-hub/devrel in a new comment on this issue if 30 days have passed since my pull request was opened and I have not received a response from the extension's maintainer.

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