Use ResourceReadyEvent to avoid blocking startup importing redis databases

[davidfowl]

Replaces #6141

Description

This pull request focuses on ensuring that all endpoints are allocated before attempting to import databases in Redis-related operations. It introduces a mechanism to wait for the allocation of endpoints and modifies the test to verify this behavior.

Improvements to Redis resource handling:

• src/Aspire.Hosting.Redis/RedisBuilderExtensions.cs: Added a TaskCompletionSource to wait for all endpoints to be allocated before importing databases. Subscribed to AfterEndpointsAllocatedEvent to signal when endpoints are ready. [1] [2]

Test updates:

• tests/Aspire.Hosting.Redis.Tests/RedisFunctionalTests.cs: Modified the test to use a TaskCompletionSource to wait for the ResourceReadyEvent instead of directly waiting for the resource state to be running. This ensures that the database import process is verified correctly.

Fixes #6136
Fixes #6099

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
  • No
• Did you add public API?
  • Yes
    • If yes, did you have an API Review for it?
      • Yes
      • No
    • Did you add <remarks /> and <code /> elements on your triple slash comments?
      • Yes
      • No
  • No
• Does the change make any security assumptions or guarantees?
  • Yes
    • If yes, have you done a threat model and had a security review?
      • Yes
      • No
  • No
• Does the change require an update in our Aspire docs?
  • Yes
    • Link to aspire-docs issue:
  • No

Microsoft Reviewers: Open in CodeFlow

[mitchdenny]

Good technique for jumping across to a non-blocking event. LGTM.