Add DeleteRamp to MigrationConfig for independent delete ramp control

[beijxu]

Summary

Add a new DeleteRamp inner class to MigrationConfig, decoupling delete operations from the existing WriteRamp. This allows independent ramp control for deletes during storage backend migration.

Changes

• Added DeleteRamp class with fields: forceDisableDualDelete, dualDeleteAsyncPct, dualDeleteSyncPctNonStrict, dualDeleteSyncPctStrict, deleteOnlyToSecondary
• Added deleteRamp field to MigrationConfig (nullable, defaults to null)
• Preserved 4-arg constructor for backward compatibility with existing callers
• @JsonIgnoreProperties(ignoreUnknown = true) on DeleteRamp for forward compatibility

Backward Compatibility

• Old JSON without deleteRamp deserializes successfully (getDeleteRamp() returns null)
• Old code without DeleteRamp ignores the new field via @JsonIgnoreProperties(ignoreUnknown = true) on MigrationConfig
• Existing 4-arg MigrationConfig constructor still works (delegates with deleteRamp=null)

Testing Done

• MigrationConfigTest: constructor, serialization round-trip, unknown field tolerance, backward-compat deserialization without deleteRamp, equals/hashCode, default constructor, 4-arg constructor compat
• BackwardsCompatibilityTest: updated field set expectations, added snapshot-without-deleteRamp test, all existing snapshot tests pass unchanged

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 37 lines in your changes missing coverage. Please review.
✅ Project coverage is 38.12%. Comparing base (52ba813) to head (b0e750c).
⚠️ Report is 406 commits behind head on master.

Files with missing lines	Patch %	Lines
...java/com/github/ambry/account/MigrationConfig.java	0.00%	37 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (52ba813) and HEAD (b0e750c). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (52ba813)	HEAD (b0e750c)
	3	2

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3273       +/-   ##
=============================================
- Coverage     64.24%   38.12%   -26.12%     
+ Complexity    10398     6463     -3935     
=============================================
  Files           840      938       +98     
  Lines         71755    80355     +8600     
  Branches       8611     9671     +1060     
=============================================
- Hits          46099    30639    -15460     
- Misses        23004    47225    +24221     
+ Partials       2652     2491      -161     

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