Add eager standalone activity start API

[wenlong-gu]

Task overview

Part 1 of 3 for ACT-1118, adding eager start for Standalone Activities (SAA).

Today, an SAA start must create the activity, enqueue a dispatch task, send it
through Matching, and wait for a worker poll. This adds queue latency even when
the caller has a ready local worker.

The complete solution lets an eligible StartActivityExecution request return
the first Activity task directly in its response. The server persists the
activity before returning that task and skips only the initial Matching dispatch.

For safety, an eager task is returned only for a newly created execution.
Same-request-ID retries and conflicts such as USE_EXISTING return the
existing execution without re-delivering the task, because Activity code may
have external side effects.

Scope of this PR

Adds the additive public API contract:

• StartActivityExecutionRequest.request_eager_execution
• StartActivityExecutionResponse.eager_activity_task

There is no runtime server behavior in this PR.

Related PRs

• API-Go: Add eager standalone activity start Go API api-go#319 updates this proto
  submodule and regenerates the Go API types.
• Temporal: Add eager start support for standalone activities temporal#12349 implements
  request eligibility, the eager state transition, task construction,
  retry/conflict safety, and tests.

Validation

• API linter passes.

Benchmark context

This contract-only PR has no runtime behavior. The Temporal implementation was
measured locally in an in-process server test cluster over 200 paired runs:

Path	p50	p95	p99
Normal SAA	2.372 ms	58.654 ms	88.855 ms
Eager SAA	1.588 ms	5.028 ms	7.362 ms

The measurement covers server-side task delivery only; it excludes SDK worker
scheduling, user Activity execution, network latency, and production load.

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[Copilot]

Copilot review overview

🟢 Approval recommended

The fields are backward-compatible additions using unused tags and an existing message type, with no blocking issues identified.

Review effort: Balanced
Findings: None

What changed in this PR

Adds the public API contract for eager standalone Activity starts, allowing callers to request direct task delivery. Server behavior is deferred to a separate PR.

Changes:

• Adds request_eager_execution to the start request.
• Adds eager_activity_task to the response, documented for newly started activities only.

File	Description
temporal/​api/​workflowservice/​v1/​request_response.proto	Adds eager-start request and response fields.

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