fix(sqlparser): correctly parse delimiters inside dollar-quoted string

[Standing-Man]

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What's changed and what's your intention?

• Closes bug(sqlparser): dollar-quoted string cannot be nested #14504
• Closes feat: support Dollar-Quoted String Constants #14763.

problem: The current sqlparser behavior incorrectly terminates or errors on nested dollar-quote-like content inside a dollar-quoted string, instead of handling it as literal content.

The implementation now treats all characters inside both untagged and tagged dollar-quoted strings as literal string content.

Checklist

• I have written necessary rustdoc comments.
• I have added necessary unit tests and integration tests.
• I have added test labels as necessary.
• I have added fuzzing tests or opened an issue to track them.
• My PR contains breaking changes.
• My PR changes performance-critical code, so I will run (micro) benchmarks and present the results.
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Documentation

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Release note

[Standing-Man]

Hi @chenzl25 and @xiangjinwu, could you please take a look when you have a moment?