fix(sqlsmith): use encoded string as input to decode

[kwannoel]

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What's changed and what's your intention?

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
• My PR contains breaking changes. (If it deprecates some features, please create a tracking issue to remove them in the future).
• All checks passed in ./risedev check (or alias, ./risedev c)
• My PR changes performance-critical code. (Please run macro/micro-benchmarks and show the results.)

• My PR contains critical fixes that are necessary to be merged into the latest release. (Please check out the details)

Documentation

• My PR needs documentation updates. (Please use the Release note section below to summarize the impact on users)

Release note

If this PR includes changes that directly affect users or other significant modifications relevant to the community, kindly draft a release note to provide a concise summary of these changes. Please prioritize highlighting the impact these changes will have on users.

[codecov]

Codecov Report

Merging #13191 (6854dec) into main (84d27ba) will decrease coverage by 0.07%.
Report is 1 commits behind head on main.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main   #13191      +/-   ##
==========================================
- Coverage   68.23%   68.16%   -0.07%     
==========================================
  Files        1506     1506              
  Lines      255354   255354              
==========================================
- Hits       174235   174058     -177     
- Misses      81119    81296     +177     

Flag	Coverage Δ
rust	68.16% <ø> (-0.07%)	⬇️

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see 19 files with indirect coverage changes

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