Add lifecycle hooks and configs for custom partial-upsert mergers

[xiangfu0]

Motivation

Custom partial-upsert mergers can own row merge behavior, but they currently cannot receive implementation-specific configuration or validate and normalize records that have no previously stored row. Plugins also need an opt-in way to validate schema changes against their table configuration.

Changes

• Add optional partialUpsertMergerConfigs to UpsertConfig.
• Add a backward-compatible PartialUpsertMerger.prepare(GenericRow) hook.
• Invoke prepare() for every partial-upsert record before previous-row lookup and update, including the first record for a primary key.
• Reject merger configs without a custom merger class and continue rejecting combinations of custom mergers with built-in per-column strategies.
• Treat a custom merger class and its configs as immutable on existing tables, preventing replicas from using mixed semantics while handlers remain initialized with old configuration.
• Add an opt-in schema-validation phase to TableConfigValidator and invoke registered validators during schema writes.

Compatibility

Existing custom mergers remain compatible because prepare() defaults to a no-op, and existing table configs need not define merger configs.

This intentionally tightens table-update validation: adding, removing, or changing a custom merger class/config on an existing table is rejected. Such handlers are initialized at table startup, so accepting live mutations could give replicas different merge semantics.

Implementations must not change primary-key or comparison-column values from prepare() because RecordInfo has already been constructed.

Tests

• 107 focused tests passed across pinot-spi and pinot-segment-local, including UpsertConfigTest, TableConfigUtilsTest, PartialUpsertHandlerTest, BasePartitionUpsertMetadataManagerTest, TableConfigValidatorRegistryTest, and SchemaUtilsTableConfigValidatorTest.
• Affected-module formatting, Checkstyle, license checks, and git diff --check passed.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.17647% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.64%. Comparing base (d08ac91) to head (7ed3c9d).

Files with missing lines	Patch %	Lines
...he/pinot/segment/local/utils/TableConfigUtils.java	85.71%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #19422      +/-   ##
============================================
- Coverage     67.67%   67.64%   -0.03%     
  Complexity     1430     1430              
============================================
  Files          3488     3490       +2     
  Lines        224395   224430      +35     
  Branches      35422    35428       +6     
============================================
- Hits         151867   151826      -41     
- Misses        60488    60577      +89     
+ Partials      12040    12027      -13     

Flag	Coverage Δ
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-25	67.64% <91.17%> (-0.03%)	⬇️
lane-a	100.00% <ø> (ø)
lane-b	0.00% <ø> (ø)
temurin	67.64% <91.17%> (-0.03%)	⬇️
unittests	67.64% <91.17%> (-0.03%)	⬇️
unittests1	57.78% <26.47%> (-0.01%)	⬇️
unittests2	39.36% <91.17%> (-0.02%)	⬇️

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