Call WalDataToDDLEvent once per event in the processor chain

[kvch]

Description

Every processor in the chain (filter, injector, transformer, and the postgres DDL adapter) independently called WalDataToDDLEvent on the same event, and each call ran json.Unmarshal([]byte(d.Content)).
From now on parsing result is cached. So there is no need for the expensive json.Unmarshals.

Cachingthe parse on wal.Data reduces 5 json.Unmarshal passes per event down to 1 (+4 cached hits), cutting time and allocations ~74%. The gain scales with how many processors in the chain call WalDataToDDLEvent on the same event.

Metric	main (baseline)	this pr	Δ vs main
Time	6.669µ	1.747µ	−73.80%
Bytes/op	5.104Ki	1.321Ki	−74.12%
Allocs/op	25.000	6.000	−76.00%

Type of Change

Please select the relevant option(s):

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🔧 Refactoring (no functional changes)
• ⚡ Performance improvement
• 🧪 Test coverage improvement
• 🔨 Build/CI changes
• 🧹 Code cleanup

Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed
• All existing tests pass

Checklist

• Code follows project style guidelines
• Self-review completed
• Code is well-commented
• Documentation updated where necessary

[github-actions]

Merging this branch will increase overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/xataio/pgstream/pkg/snapshot/generator/postgres/data	78.75% (ø)
github.com/xataio/pgstream/pkg/wal	72.31% (+0.65%)	👍
github.com/xataio/pgstream/pkg/wal/listener/kafka	69.57% (ø)
github.com/xataio/pgstream/pkg/wal/processor/filter	94.00% (ø)

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/xataio/pgstream/pkg/wal/wal_data.go	0.00% (ø)	11	0	11
github.com/xataio/pgstream/pkg/wal/wal_ddl.go	71.15% (+1.77%)	52 (+3)	37 (+3)	15	👍

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/xataio/pgstream/pkg/snapshot/generator/postgres/data/pg_snapshot_generator_test.go
• github.com/xataio/pgstream/pkg/wal/listener/kafka/wal_kafka_reader_test.go
• github.com/xataio/pgstream/pkg/wal/processor/filter/wal_filter_test.go
• github.com/xataio/pgstream/pkg/wal/wal_ddl_test.go