[Quinn-With-Two-Ns]

What changed?

• Expose a primary temporalNexusClient bean for the root namespace and <alias-or-namespace>NexusClient beans for non-root namespaces.
• Support namespace-specific data converters and TemporalOptionsCustomizer<NexusClientOptions.Builder> beans, with automatic interceptor registration for the root client.
• Add integration tests and concise documentation. Per-service clients and YAML configuration are out of scope.

Why?

Make it easy to inject and reuse a process-lifetime Nexus client, following the existing WorkflowClient and ScheduleClient Spring integration.

Closes #3122.

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Note

Low Risk
Additive Spring Boot wiring and new beans; constructor signature changes are internal to the autoconfigure module with tests covering behavior.

Overview
Adds Spring Boot auto-configuration for NexusClient, mirroring the existing WorkflowClient / ScheduleClient integration.

Applications get a primary temporalNexusClient for the root namespace and <alias-or-namespace>NexusClient singletons for configured non-root namespaces. Clients are built through ClientTemplate and a new NexusClientOptionsTemplate (namespace, per-namespace DataConverter, ordered TemporalOptionsCustomizer<NexusClientOptions.Builder> beans). Root-namespace NexusClientInterceptor beans are wired automatically; non-root namespaces use namespace-prefixed customizers only (interceptors via customizer, not auto-discovery).

NonRootBeanPostProcessor registers the new beans alongside other per-namespace clients. Deprecated ClientTemplate / NonRootNamespaceTemplate constructors without plugin/Nexus parameters are removed. CHANGELOG, README, and NexusClientAutoConfigurationTest cover injection, multi-namespace connections, customizers, and root interceptors.

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