disagg: adapt GC merge fan-in after S3 read failures (#10800)

[ti-chi-bot]

This is an automated cherry-pick of #10800

What problem does this PR solve?

Issue Number: close #10798, ref #10794

Problem Summary:

Background GC merges can still fan in too many segments after bounded S3 stream retries are exhausted. That keeps a single GC merge exposed to long remote-read windows, while medium forward seeks still prefer draining the existing stream instead of reopening from the target offset.

What is changed and how it works?

33e66a4a07 disagg: tighten S3 forward seek reopen threshold
bd070ca5fe disagg: classify retry-exhausted S3 reads
7cbe1147ae disagg: adapt GC merge fan-in after S3 errors
085cb158be disagg: clarify S3 retry exhaustion flow
1f87ade691 disagg: narrow GC cap adjustment triggers

• tighten the S3 forward-seek reopen threshold from 128 KiB to 64 KiB and update the boundary tests
• let S3RandomAccessFile::read and S3RandomAccessFile::seek throw ErrorCodes::S3_ERROR after the bounded stream retry budget is exhausted, with doc comments for the public APIs and concise comments around the retry-exhaustion branch
• add a table-level gc_mergeable_segments_cap so background GC merge fan-in is capped and can be reduced after S3_ERROR
• only reduce the cap when gcTrySegmentMerge fails with S3_ERROR, and recover the cap only after checkSegmentUpdate succeeds
• add failpoints and unit tests for cap limiting, S3-error-triggered reduction, recovery, and non-S3 failures

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

Summary by CodeRabbit

• New Features

  • Background garbage collection now dynamically adjusts merge concurrency based on S3 error conditions, automatically recovering when operations succeed.
  • S3 operations now throw explicit error codes when retry budgets are exhausted, improving error handling clarity.

• Chores

  • Adjusted S3 stream forwarding threshold from 128KB to 64KB for optimized performance.

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-04-14 05:22:11.654020543 +0000 UTC m=+1452136.859380610: ☑️ agreed by JaySon-Huang.
• 2026-04-14 05:36:43.09680412 +0000 UTC m=+1453008.302164177: ☑️ agreed by CalvinNeo.

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: CalvinNeo, JaySon-Huang, JinheLin, kolafish

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Needs approval from an approver in each of these files:

• OWNERS [CalvinNeo,JaySon-Huang,JinheLin]

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