refactor(generator): convert PGX and batch functions to poet statements

Replace string building with structured poet AST types for:
- addQueryOnePGX, addQueryManyPGX, addQueryExecPGX
- addQueryExecRowsPGX, addQueryExecResultPGX
- addCopyFromCodeMySQL
- batch functions (batchexec, batchmany, batchone)

This eliminates manual string building with fmt.Fprintf and
WriteString calls in favor of structured poet.If, poet.For,
poet.Assign, poet.Defer, and other statement types.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: claude