[fix](balance) Fix PartitionRebalancer generating invalid moves to BEs without required storage medium

[deardeng]

When tablet_rebalancer_type=Partition, adding a new BE with only HDD disks to a cluster where tables use SSD storage medium causes the PartitionRebalancer to generate invalid moves (SSD tablets -> HDD-only BE), resulting in infinite "paths has no available balance slot: []" scheduling failures.

Root cause:

1. In LoadStatisticForTag.init(), beByTotalReplicaCount for each medium includes ALL available BEs without checking hasMedium(). This causes the greedy algorithm to consider HDD-only BEs as valid destinations for SSD tablets.
2. In LocalTabletInvertedIndex.buildPartitionInfoBySkew(), the countMap initialization uses all availableBeIds without medium filtering, so HDD-only BEs get counted with 0 replicas for SSD partitions, making them appear as the "least loaded" and preferred move target.

Fix:

1. Add hasMedium() filter in LoadStatisticForTag.init() when building beByTotalReplicaCount, so only BEs that actually have the required medium are considered for balancing.
2. Add availableBeIdsByMedium parameter to buildPartitionInfoBySkew() and use it to initialize countMap with only medium-matching BEs, preventing BEs without the required medium from appearing in the skew calculation.

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

Release note

None

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

• Behavior changed:

  • No.
  • Yes.

• 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

[Thearas]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[deardeng]

run buildall

[hello-stephen]

FE UT Coverage Report

Increment line coverage 93.33% (14/15) 🎉
Increment coverage report
Complete coverage report

[yujun777]

LGTM

[github-actions]

PR approved by anyone and no changes requested.

[deardeng]

run nonConcurrent

[github-actions]

PR approved by at least one committer and no changes requested.