feat(frontend): purification for source def sql

[BugenZhao]

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What's changed and what's your intention?

Sequel to #19949. Extract the purification snippet to reuse for CREATE SOURCE.

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.
• My PR contains critical fixes that are necessary to be merged into the latest release.

Documentation

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Release note

[BugenZhao]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• refactor(frontend): reuse def purification for assembling cdc table def when auto schema change #19997
• feat(frontend): "purify" column default value  #19993
• feat(frontend): purification for source def sql #19965 👈 (View in Graphite)
• feat(frontend): initially introduce table def sql purification #19949
• refactor(frontend): extract code for parsing table def sql into single stmt #19948
• main

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