fix(pgcopy): return typed error instead of panicking on extra columns from query:

[SAY-5]

What

Fixes #432.

pgcopy.(*Row).Decode walked past the end of columnPos whenever the COPY stream contained more columns than the row was constructed for. The dump goroutine then crashed with:

panic: runtime error: index out of range [5] with length 5
github.com/greenmaskio/greenmask/internal/db/postgres/pgcopy.(*Row).Decode

This fired any time a transformation's query: selected extra columns beyond the table's declared schema.

Fix

Added an explicit length check on the fixed-size path: if a non-dynamic row sees idx >= len(columnPos), return a new ErrTupleSizeMismatch sentinel wrapped with the offending byte offset so the caller can surface a useful message. The dynamic path (UseDynamicSize) is unchanged.

(*TransformationPipeline).Dump already wraps Decode errors with error decoding copy line: %w, so the existing error path carries the message all the way out without any code changes at the call site.

Tests

Added TestDecode_extraColumnsTupleSizeMismatch: constructs a 2-column row, feeds it 5 columns over the wire, asserts ErrTupleSizeMismatch is returned. On master this panics; with the fix it returns the typed error.

Verification

Locally on macOS, go 1.26.2:

• gofmt -s -l internal/db/postgres/pgcopy/: clean
• go vet ./internal/db/postgres/pgcopy/...: clean
• go test -race -count=1 ./internal/db/postgres/pgcopy/...: pass

Closes #432

[wwoytenko]

Hi! Thank you for your contribution.

Could you provide a config that can reproduce a behavior related to this change? So I could test it in playground.

[wwoytenko]

Have you tested the whole dump/restore cycle?