Add SAA support to TemporalOperationHandler

[VegetarianOrc]

What was changed

Added support for Activity-backed Nexus operations in the TemporalOperationHandler.

• TemporalNexusClient.startActivity(): starts a standalone activity as an async Nexus operation, propagating callbacks, request ID, and inbound/outbound links automatically
• NexusTypedActivityClient: strongly-typed variant (via .typedActivity()) for type-safe activity name, args, and result
• New ActivityOperationToken (token type 2) with aid/rid fields to identify activity-backed operations
• cancelActivity option on TemporalOperationHandlerOptions with a default implementation that cancels the underlying activity
• Extracted shared startWithNexusOptions helper to deduplicate link conversion, callback attachment, and back-link propagation between workflow and activity start paths
• Moved ActivityOptions and related types (ActivityOptionsFor, ActivityName, ActivityResult) to a new activity-options.ts file and re-exported from @temporalio/client

Why?

Temporal Nexus operations should be able to start and cancel activities as well as workflows, using a consistent operation token and backing-operation model across both paths.

Checklist

1. How was this tested:

• Expanded test-nexus-temporal-operation.ts with integration tests covering: activity start, idempotent re-start (conflict handling), cancellation, and bad-options error surfacing as BAD_REQUEST rather than internal retries
• Token encode/decode round-trip tests in test-nexus-token-helpers.ts

[Evanthx]

LGTM