Bump patch version from 13.3.0 to 13.3.1

[Copilot]

Description

Starts 13.3 servicing builds by moving the release/13.3 branch from 13.3.0 to 13.3.1, matching the initial patch-bump pattern used on release/13.2.

• Repo versioning

  • Updates eng/Versions.props to set the branch patch version to 13.3.1.

• Out-of-repo test assets

  • Updates tests/Shared/RepoTesting/Aspire.RepoTesting.targets so Helix/out-of-repo test runs import Aspire.AppHost.Sdk version 13.3.1.

• Helix package references

  • Updates tests/Shared/RepoTesting/Directory.Packages.Helix.props so the pinned Aspire package versions used for archived/out-of-repo test execution move from 13.3.0 to 13.3.1.

Example of the servicing bump applied in this PR:

<PatchVersion>1</PatchVersion>
<Import Project="Sdk.props" Sdk="Aspire.AppHost.Sdk" Version="13.3.1" />

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.dev issue:
      • New issue
  • No