[fix](cloud) modify delete_bitmap_store_v2_max_bytes_in_fdb

[mymeiyi]

In test, a 64 KiB threshold reduces performance by 6% for large bitmaps, while 128 KiB shows no measurable slowdown. And consider split_size in blob_put, set to 90000.

[hello-stephen]

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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?
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[Copilot]

🟢 Approval recommended

The change is a straightforward config default adjustment with a minor maintainability nit around documenting the rationale/units for the new magic number.

Pull request overview

This PR adjusts the default Cloud BE configuration for when delete-bitmap Store V2 data should be kept directly in FoundationDB versus spilled to external storage, choosing a larger threshold intended to reduce overhead for large delete bitmaps and align with existing blob chunking behavior.

Changes:

• Updated delete_bitmap_store_v2_max_bytes_in_fdb default from 1024 to 90000.
• Added an inline comment noting the intended alignment with blob_put split sizing.

Review Checkpoints (per Doris checklist)

• Goal & correctness: The change matches the stated intent of increasing the threshold to avoid performance regressions from too-small limits; no functional logic changes beyond default behavior.
• Scope/focus: Small and focused (single config default + comment).
• Concurrency / lifecycle: Not applicable (config definition only).
• Config behavior: This is a default behavior change (more delete-bitmaps may be stored in FDB by default); the chosen number should be clearly justified in-code because it impacts operational characteristics.
• Test coverage: Existing regression suites reference this config, but they set it explicitly in test sessions; the new default itself is not directly covered by those tests.

File summaries

File	Description
be/src/cloud/config.cpp	Raises the default delete-bitmap Store V2 “max bytes in FDB” threshold to 90,000 and documents intended alignment with blob_put chunking.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Lite

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[mymeiyi]

run buildall

[hello-stephen]

BE UT Coverage Report

Increment line coverage 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	63.03% (29554/46891)
Line Coverage	48.09% (310051/644700)
Region Coverage	43.68% (250517/573592)
Branch Coverage	45.25% (116533/257553)

[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 100% (0/0) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	76.29% (34648/45417)
Line Coverage	61.39% (391250/637337)
Region Coverage	57.55% (328847/571408)
Branch Coverage	58.38% (149930/256814)