Standalone Activities start delay

[GregoryTravis]

Add start_delay: option to Client#start_activity / #execute_activity for Standalone Activities.

Mirrors the existing Client#start_workflow(start_delay:) semantics on the activity side. When set, the server creates the activity immediately (visible in describe/list responses with status RUNNING, run state SCHEDULED) but defers dispatch to a worker until the delay elapses. Marked experimental per existing project convention.

Spec

• 1-pager: https://app.notion.com/p/3308fc56773880bc9a40d6886275c0ef

Public API surface

client.start_activity(
  MyActivity, ...,
  id: 'act-1', task_queue: 'tq', start_to_close_timeout: 60,
  start_delay: 30.0   # ← new; seconds, nullable, non-negative
)

• Added start_delay: kwarg (default nil) to Temporalio::Client#start_activity and #execute_activity.
• Added start_delay field to Client::Interceptor::StartActivityInput so interceptors can observe / modify it.
• Client-side validation: raises ArgumentError, 'start_delay must be non-negative' for negative values (matches dotnet's ArgumentException). nil / 0 are accepted as "no delay."
• Wired onto StartActivityExecutionRequest.start_delay (proto field 22, already present in generated bindings — no proto regeneration needed).

Behavior covered by tests

#	Test	Verifies
1	test_start_activity_rejects_negative_start_delay	ArgumentError raised pre-RPC for negative values (mirrors dotnet's NegativeStartDelay_Throws)
2	test_start_activity_with_start_delay_dispatches_after_delay	last_started_time - schedule_time >= delay - 500ms
3	test_start_activity_with_zero_start_delay_dispatches_immediately	start_delay: 0 behaves like no delay
4	test_start_activity_with_nil_start_delay_dispatches_immediately	start_delay: nil behaves like no delay
5	test_cancel_during_start_delay_transitions_to_canceled_immediately	Server cancels the activity within the delay window without waiting for dispatch
6	test_terminate_during_start_delay_transitions_to_terminated_immediately	Same for terminate
7	test_start_delay_extends_schedule_to_start_timeout	ScheduleToStart timeout window begins after the delay (activity with schedule_to_start_timeout: 0.5s and start_delay: 1.0s dispatches successfully)
8	test_start_delay_extends_schedule_to_close_timeout	ScheduleToClose timeout window begins after the delay (same shape)

Test environment

test/test.rb:

• Bumped dev_server_download_version from v1.7.0 to v1.7.1-standalone-nexus-operations to include SAA supprt.
• Added --dynamic-config-value activity.startDelayEnabled=true to the dev-server args.

[chris-olszewski]

I realize we're still developing what the CHANGELOG should look like, but I think we want to keep it fairly terse.