fix: resolve panic in measure block merger for overlapping timestamps

[hanahmily]

• If this pull request closes/resolves/fixes an existing issue, replace the issue number. Fixes [Bug] Banyandb standlone mode query data error then db can't start skywalking#13604.
• Update the CHANGES log.

[Copilot]

Pull request overview

This PR fixes a critical panic in the measure block merger that occurred when merging blocks with overlapping timestamps. The issue manifested as an "index out of range [-1]" error when the merger attempted to access an array element at index -1.

Key Changes:

• Added bounds check in mergeTwoBlocks to prevent array index out of bounds panic
• Added optimization to swap blocks upfront when the right block starts with a smaller timestamp
• Comprehensive test coverage including edge case tests and file-based merge scenarios

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
banyand/measure/merger.go	Fixed panic by adding bounds check (i > left.idx) before accessing left.timestamps[i-1] and added optimization to swap blocks if right starts with smaller timestamp
banyand/measure/merger_test.go	Added Test_mergeTwoBlocks_edgeCase to test the specific panic scenario, added Test_mergeParts_fileBased with 4 comprehensive sub-tests, and added helper functions for generating test data
CHANGES.md	Added bug fix entry describing the panic fix for measure block merger

The changes are well-implemented with proper edge case handling and comprehensive test coverage. The fix correctly addresses the root cause by ensuring array bounds are validated before access, and the optimization helps reduce unnecessary swaps during the merge process. The test cases thoroughly cover various scenarios including overlapping timestamps, block splitting when exceeding maxBlockLength, and multiple blocks per part.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.11%. Comparing base (3530dd9) to head (dca2e96).
⚠️ Report is 83 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #914      +/-   ##
==========================================
+ Coverage   45.97%   46.11%   +0.14%     
==========================================
  Files         328      368      +40     
  Lines       55505    56572    +1067     
==========================================
+ Hits        25520    26091     +571     
- Misses      27909    28015     +106     
- Partials     2076     2466     +390     

Flag	Coverage Δ
banyand	49.29% <100.00%> (?)
bydbctl	81.91% <ø> (?)
fodc	90.65% <ø> (?)
integration-distributed	80.00% <ø> (?)
pkg	29.07% <ø> (?)

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