fix(persistent-subscriptions): reply not ready before startup

[yordis]

• Persistent subscription clients need a retryable response while the service is loading after leadership changes.
• HTTP callers need a service-unavailable response instead of an unexpected server error when the subsystem is not ready.

[cursor]

PR Summary

Medium Risk
Changes wire/HTTP behavior for persistent subscription management and connection requests when the subsystem hasn’t started, which could affect client retry/handling paths. Scope is contained and covered by new tests, but touches multiple request handlers and HTTP response mapping.

Overview
Persistent subscription operations now return a retryable ClientMessage.NotHandled(NotReady) instead of silently doing nothing when the subsystem hasn’t started (create/update/delete, connect, read-next, replay-parked).

HTTP endpoints for persistent subscriptions and stats now treat NotHandled responses as proper service-level errors (notably mapping NotReady to 503 Service Unavailable) by reusing a shared Configure.HandleNotHandled helper (made public).

Adds PersistentSubscriptionServiceNotReadyTests to assert NotReady replies across the affected message types.

^{Reviewed by Cursor Bugbot for commit 6213cf1. Bugbot is set up for automated code reviews on this repo. Configure here.}

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 2c610390-3550-47ba-865c-ff9e841baebc

📥 Commits

Reviewing files that changed from the base of the PR and between 270f627 and 6213cf1.

📒 Files selected for processing (4)

• src/EventStore.Core.Tests/Services/PersistentSubscription/PersistentSubscriptionServiceNotReadyTests.cs
• src/EventStore.Core/Services/PersistentSubscription/PersistentSubscriptionService.cs
• src/EventStore.Core/Services/Transport/Http/Configure.cs
• src/EventStore.Core/Services/Transport/Http/Controllers/PersistentSubscriptionController.cs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch yordis/fix-persistent-subscriptions-not-ready

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, have a team admin enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 1091a08. Configure here.}