refactor: Extract pg_stat_statements column logic to helper

[jssmith]

Summary

Follow-up to #132: Refactors version-dependent column logic into a reusable helper function.

What Changed

1. Added _get_pg_stat_statements_columns() helper

Extracts duplicated version logic from both get_top_queries_by_time() and get_top_resource_queries() into a single helper function.

Before: Version branching duplicated in 2 places
After: Single source of truth

Returns a PgStatStatementsColumns dataclass with all version-appropriate column names.

2. Added comprehensive documentation

def _get_pg_stat_statements_columns(pg_version: int) -> PgStatStatementsColumns:
    """Get pg_stat_statements column names based on PostgreSQL version.

    PostgreSQL 13 introduced pg_stat_statements v2.0 with breaking changes:
    - Renamed timing columns: *_time → *_exec_time (total_time → total_exec_time, etc.)
    - Added wal_bytes column for write-ahead log tracking

    This function provides version-appropriate column names to ensure compatibility
    with both old (PG ≤ 12) and new (PG ≥ 13) versions.

    See: https://www.postgresql.org/docs/13/release-13.html#id-1.11.6.11.4
    ...

Benefits

✅ DRY - Eliminates duplication
✅ Maintainable - One place to update if future PG versions change
✅ Testable - Helper can be unit tested independently
✅ Type-safe - Dataclass prevents typos in column names
✅ Self-documenting - Clear structure shows version differences

Testing

• ✅ All existing unit tests pass (8/8)
• ✅ No behavior changes - pure refactor
• ✅ Lint and type checks pass

Files Changed

• src/postgres_mcp/top_queries/top_queries_calc.py - Extracted helper function

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