Add standalone Nexus operations client

[VegetarianOrc]

NOTE FOR REVIEWERS: This PR is pending the actual release of the API and Server and will not be merged until those are complete.

What was changed

Add a standalone Nexus operations client to the TypeScript SDK.

• New NexusClient sub-client on Client, accessible via client.nexus.
• NexusClient.createServiceClient({ endpoint, service }) returns a typed NexusServiceClient built from a nexus-rpc
  ServiceDefinition
• NexusClient.getHandle(operationId, { runId? }) to rehydrate a handle for an existing operation.
• NexusOperationHandle supports result(), describe(), cancel(), and terminate().
• Listing / counting APIs: NexusClient.list(options) (async iterable) and NexusClient.count(options).
• New types in packages/client/src/nexus-types.ts
• Interceptor support via ClientInterceptors.nexus / NexusClientInterceptor with inputs for start, get-result, describe, cancel, terminate, list, and count.
• New error classes: NexusOperationFailureError, NexusOperationAlreadyStartedError, NexusOperationNotFoundError.
• Re-exports wired in packages/client/src/index.ts

Checklist

1. How was this tested:
   • Added packages/test/src/test-nexus-standalone.ts with integration tests exercising startOperation / executeOperation, result, describe, cancel, terminate, list, count, id-reuse and id-conflict policies, search attributes, and interceptor.
   • Added packages/test/src/test-nexus-standalone-types.ts with compile-time assertions that createServiceClient produces correctly typed start/execute signatures and handles generic inference from nexus.OperationDefinition.
2. Any docs updates needed?
   - Yes SANO will need documentation for all languages.