Add support for parsing COPY statements from STDIN without a semicolon

[LucaCappelletti94]

This PR fixes PostgreSQL COPY ... FROM STDIN parsing when the statement ends at EOF without a trailing semicolon.

This is one of several failure families I have found in the course of the correctness benchmarks I am running based on resources such as the PostgreSQL regression SQL suite.

Before this change, sqlparser rejected valid inputs such as:

COPY bitwise_test FROM STDIN NULL 'null'

with:

ParserError("Expected: ;, found: EOF")

Root Cause

parse_copy unconditionally required ; for CopyTarget::Stdin before reading optional inline payload rows.

Fix

For COPY ... FROM STDIN:

• accept immediate EOF as a valid terminator when no inline payload is present
• keep existing behavior for semicolon + payload parsing unchanged

Tests

Added/updated PostgreSQL regression coverage:

• parse_copy_from_stdin_without_semicolon
• parse_copy_from_stdin_without_semicolon_variants

The variants cover multiple FROM stdin shapes without trailing semicolon (columns, legacy options, mixed casing), and assert they parse as Statement::Copy with target: Stdin and empty payload.