Add eager standalone activity start Go API

[wenlong-gu]

Task overview

Part 2 of 3 for ACT-1118, adding eager start for Standalone Activities (SAA).

The overall feature lets an eligible StartActivityExecution request return
its first Activity task directly, avoiding the initial Matching round trip.
The server returns an eager task only for a newly created execution. It does
not re-deliver that task for same-request-ID retries or conflicts, because
Activity code can have external side effects.

Scope of this PR

Updates the proto/api submodule to API PR #876 and regenerates the Go API
types for:

• StartActivityExecutionRequest.RequestEagerExecution
• StartActivityExecutionResponse.EagerActivityTask

This PR contains no Temporal server behavior.

Related PRs

• API contract: Add eager standalone activity start API api#876
• Temporal server: Add eager start support for standalone activities temporal#12349 consumes
  this generated API, implements eager SAA state transitions and response
  construction, and adds functional coverage.

Validation

• CGO_ENABLED=0 go test ./workflowservice/v1 -run '^$'

Benchmark context

This generated-API-only PR has no runtime behavior. The final Temporal server
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 generated types and protobuf descriptors consistently implement the intended API contract.

Review effort: Balanced
Findings: None

What changed in this PR

Adds generated Go API support for eager standalone Activity execution.

Changes:

• Adds the eager-execution request flag and response task.
• Updates protobuf descriptors and accessors.

File	Description
workflowservice/​v1/​request_response.pb.go	Adds generated eager Activity fields and descriptor metadata.

Files not reviewed (1)

• workflowservice/v1/request_response.pb.go: Generated file

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