[fix](cloud) Fix group commit routing for virtual compute groups

[Yukang-Lian]

What problem does this PR solve?

Issue Number: None

Related PR: #61555

Problem Summary:

Group commit resolves a virtual compute group to its active physical compute group when collecting backend candidates. However, it previously used the original virtual name for the backend cache and membership validation. Because each backend carries its physical compute group name, all healthy candidates were rejected and group commit stream loads failed with No suitable backend.

This change resolves the physical compute group once on the master and consistently uses that request-level snapshot for cache lookup, candidate selection, and backend membership validation. A failover therefore uses a separate physical-group cache key and cannot reuse a backend from the previous active group.

The existing VCG Docker suite now enables synchronous group commit before and after failover and explicitly exercises both the master HTTP path and follower-to-master forwarding path.

Release note

Fix group commit stream loads through virtual compute groups.

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

  Validation performed:

  • ./run-fe-ut.sh --run org.apache.doris.load.GroupCommitManagerTest (2 tests, 0 failures, 0 errors)
  • The targeted regression suite was loaded successfully by the regression framework. Its Docker execution was skipped locally because excludeDockerTest=true; CI is expected to run the actual cloud cluster scenario.

• Behavior changed:

  • No.
  • Yes. Group commit requests through a virtual compute group now route to its active physical compute group.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[hello-stephen]

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[Yukang-Lian]

/review

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