fix(django-spanner): backslash-escape string literals in quote_value

[Samin061]

quote_value inlines a string value into DDL by wrapping it in single quotes and doubling embedded quotes, but Cloud Spanner GoogleSQL does not treat '' as an escaped quote and the routine never escapes backslashes, so a value beginning with a backslash and a quote closes the literal and the trailing text is parsed as DDL. The Spanner backend sets requires_literal_defaults, so column and table comments and altered defaults reach this method. The db_default and generated-column inlining a few lines above already escape by backslashing the backslash and then the quote, so route quote_value through the same escaping.

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[gemini-code-assist]

Code Review

This pull request updates the quote_value method in django_spanner/schema.py to properly escape backslashes and single quotes for GoogleSQL string literals, and adds corresponding unit tests. The feedback suggests also escaping literal newlines and carriage returns to prevent syntax errors in Spanner.

[gemini-code-assist]

In GoogleSQL, single-quoted string literals cannot contain literal newlines (\n) or carriage returns (\r). If a string containing these characters is passed to quote_value, it will result in a syntax error in Spanner. To prevent this, we should also escape newlines and carriage returns.

Suggested change

	return "'%s'" % value.replace("\\", "\\\\").replace("'", "\\'")
	return "'%s'" % value.replace("\\", "\\\\").replace("'", "\\'").replace("\n", "\\n").replace("\r", "\\r")