[improvement] Control HNSW build chunk memory

[yx-keith]

What problem does this PR solve?

Issue Number: None

Related PR: None

Problem Summary: Reduce ANN/HNSW index build memory spikes by switching chunk sizing to a byte-aware policy, releasing temporary vector buffers more aggressively, and adding a first-pass memory budget admission path for ANN index writers.

Release note

None

Check List (For Author)

• Test: Partial
  • Unit Test / Manual test / No need to test (with reason)
  • Added/updated BE unit tests in ann_index_writer_test.cpp
  • Attempted to run ./run-be-ut.sh --run --filter=AnnIndexWriterTest, but the local environment is pinned to JDK 8 and Doris BE UT requires JDK 17, so the suite could not be executed in this workspace
• Behavior changed: Yes (ANN/HNSW build now sizes chunks by byte budget and may wait/skip/fail on configured memory admission policy)
• Does this need documentation: No

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

[hello-stephen]

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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.
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4. Which code was refactored and why was this part of the code refactored?
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