fix: preserve indentation of lines inside quoted identifiers with pretty=True

[saluc28]

Fixes #7593

Root cause

Generator.indent() splits sql on "\n" and prepends spaces to every
line unconditionally. If a quoted string or identifier contains a literal
newline, the continuation line is a content byte — not a structural line
break — so adding indentation there corrupts the value.

Fix

Track quote state while iterating the characters of each line. When the
current position is inside a quoted string ('...') or a quoted identifier
("..."), the line is appended as-is. Outside quotes, normal indentation
applies. Doubled quote characters (the SQL escape for a literal quote) are
handled with a one-step lookahead so they don't prematurely close the state.

Tests

Two assertions added to the existing test_pretty_line_breaks test:

self.assertEqual(transpile('SELECT "1\n2"', pretty=True)[0], 'SELECT\n  "1\n2"')
self.assertEqual(
    transpile('SELECT "Product\n(Foo, Bar)" AS x FROM t', pretty=True)[0],
    'SELECT\n  "Product\n(Foo, Bar)" AS x\nFROM t',
)

[saluc28]

addressed in this PR. duckdb allows any whitespace (including newlines) inside double-quoted identifiers: https://duckdb.org/docs/current/sql/dialect/keywords_and_identifiers

[georgesittas]

Hey @saluc28, thanks for the PR, but this approach is too complicated. We already have logic in the generator to avoid this issue for string literals; it was simply missing from Identifier's generator method.

Take a look at my fix, instead: 17432c6.