PMM-14594 Backup compatibility fix.

[JiriCtvrtka]

PMM-14594

FB: Percona-Lab/pmm-submodules#4368

• Fixed MySQL backup compatibility checks for MySQL 8.4 LTS by allowing Percona XtraBackup 8.4.x with MySQL 8.4.x.
• Kept existing MySQL 8.0.22+ behavior: XtraBackup 8.0.x must have the same or newer core version and stay below 8.1.
• Improved incompatible XtraBackup error messages so they explain which XtraBackup family/version should be used.
• Added unit coverage for MySQL 8.4 positive/negative compatibility cases and equivalence tests to ensure the refactor preserves existing behavior.
• Updated MySQL backup prerequisites documentation with the MySQL-to-XtraBackup compatibility matrix.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.56%. Comparing base (8cef340) to head (4077320).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5395      +/-   ##
==========================================
+ Coverage   43.55%   43.56%   +0.01%     
==========================================
  Files         414      414              
  Lines       42985    43058      +73     
==========================================
+ Hits        18721    18759      +38     
- Misses      22388    22419      +31     
- Partials     1876     1880       +4     

Flag	Coverage Δ
admin	34.78% <ø> (ø)
agent	48.80% <ø> (-0.34%)	⬇️
managed	43.02% <100.00%> (+0.15%)	⬆️
vmproxy	72.22% <ø> (ø)

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[JiriCtvrtka]

@copilot review

[it-percona-cla]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
2 out of 3 committers have signed the CLA.

✅ catalinaadam
✅ JiriCtvrtka
❌ Copilot
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[Copilot]

@copilot review

Reviewed the changes. The core logic looks correct:

• maxAlignedXtrabackupVersion (9.0) properly replaced with bounded version constants (mysql81Version, mysql84Version, mysql85Version)
• Compatibility matrix entries for pre-8.0.22 MySQL now correctly use 8.1.0 as the XtraBackup upper bound instead of 9.0
• MySQL 8.4.x → XtraBackup 8.4.x mapping is correctly implemented
• Error messages are now actionable with specific guidance per version range
• Tests cover all new paths including MySQL 8.4 + XtraBackup 8.4 compatibility

One documentation formatting issue was found and fixed in commit 1d0cd0e: the version compatibility table and the XtraBackup 8.4 note were placed outside the caution admonition block. They are now properly indented inside it.

[JiriCtvrtka]

@copilot review and verify if #5395 (comment) is addressed.