Adding mutation batcher and row assembler in cdc data generator

[shreyakhajanchi]

📝 Description

This PR introduces the MutationBatcher and RowAssembler components into the v2/cdc-data-generator module, separating row synthesis from transient buffering logic. It accumulates and chunks mutations in worker memory based on size limits partitioned per table, shard, and operation type (INSERT, UPDATE, DELETE).

Additionally, it implements FailureRecord to uniformly format serialization/writer errors into JSON lines for dead-letter queue (DLQ) storage.

Changes

Core Production Classes

• BufferKey.java [NEW]: AutoValue key structure for buffer group partitioning.
• MutationBatcher.java [NEW]: Accumulates rows, drives local thresholds, and manages transactional flushes.
• RowAssembler.java [NEW]: Pure-function helpers for updating and deleting Beam Row assembly.
• FailureRecord.java [NEW]: Uniform serialization wrapper for DLQ errors.
• DataGeneratorUtils.java [MODIFY]: Embedded hex conversion helpers (canonicalizeValue, bytesToHex).

Unit Test Suites

• MutationBatcherTest.java [NEW]: Validates batch thresholds, selective update flushes, and custom connection constraints.
• RowAssemblerTest.java [NEW]: Asserts field schema mapping rules.
• FailureRecordTest.java [NEW]: Verifies JSON conversion attributes across raw byte arrays and nulls.

[codecov]

Codecov Report

❌ Patch coverage is 82.52033% with 43 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.28%. Comparing base (5874e81) to head (74d5796).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ud/teleport/v2/templates/dofn/MutationBatcher.java	78.72%	7 Missing and 13 partials ⚠️
...cloud/teleport/v2/templates/dofn/RowAssembler.java	87.50%	7 Missing and 6 partials ⚠️
...eleport/v2/templates/utils/DataGeneratorUtils.java	63.15%	5 Missing and 2 partials ⚠️
...oud/teleport/v2/templates/utils/FailureRecord.java	88.88%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3762      +/-   ##
============================================
+ Coverage     53.16%   53.28%   +0.12%     
+ Complexity     6490     6162     -328     
============================================
  Files          1075     1079       +4     
  Lines         65233    65490     +257     
  Branches       7230     7289      +59     
============================================
+ Hits          34680    34896     +216     
- Misses        28223    28243      +20     
- Partials       2330     2351      +21     

Components	Coverage Δ
spanner-templates	72.85% <ø> (-0.01%)	⬇️
spanner-import-export	68.63% <ø> (-0.02%)	⬇️
spanner-live-forward-migration	80.98% <ø> (-0.02%)	⬇️
spanner-live-reverse-replication	77.19% <ø> (-0.02%)	⬇️
spanner-bulk-migration	91.13% <ø> (-0.01%)	⬇️
gcs-spanner-dv	85.81% <ø> (-0.02%)	⬇️

Files with missing lines	Coverage Δ
...e/cloud/teleport/v2/templates/model/BufferKey.java	100.00% <100.00%> (ø)
...oud/teleport/v2/templates/utils/FailureRecord.java	88.88% <88.88%> (ø)
...eleport/v2/templates/utils/DataGeneratorUtils.java	81.25% <63.15%> (-5.64%)	⬇️
...cloud/teleport/v2/templates/dofn/RowAssembler.java	87.50% <87.50%> (ø)
...ud/teleport/v2/templates/dofn/MutationBatcher.java	78.72% <78.72%> (ø)

... and 5 files with indirect coverage changes

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