Fix JSONB string scalar serialization

[tsg]

Description

serializeJSONBValue did not handle Go string values for jsonb/json columns. When pgx unmarshals a JSON string scalar like "FIRST", it returns a bare Go string without JSON quotes. This was passed through to CopyFrom unchanged, causing PostgreSQL to reject it as invalid JSON.

Add a string case that JSON-encodes values not already valid JSON, while preserving passthrough for pre-built JSON documents.

NOTE: opened against 0.9.x, we need to merge 0.9.x into main after releasing.

Related Issue(s)

• Fixes Snapshot fails with "invalid input syntax for type json" for JSONB columns containing JSON string scalars #743

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

[github-actions]

Merging this branch will increase overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/xataio/pgstream/pkg/wal/processor/postgres	83.83% (+0.06%)	👍

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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/processor/postgres/postgres_wal_dml_adapter.go	94.74% (+0.11%)	152 (+3)	144 (+3)	8	👍

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/wal/processor/postgres/jsonb_serialization_test.go