fix(index): encode string type to original data to enable fst regex to work

[zhongzc]

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What's changed and what's your intention?

Previously, when encoding string keys into an FST using the memcomparable format, additional prefixes and suffixes beyond the original string data were added, which could lead to inaccuracies under certain regex query conditions. This PR specifically addresses the encoding of strings: encoding is now done directly based on the original data.

Additionally, unit tests related to SstIndexCreator have been added. These tests link together both the construction and querying of indexes, increasing confidence in their correctness.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (4810c91) 85.31% compared to head (f6a3a49) 84.95%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3324      +/-   ##
==========================================
- Coverage   85.31%   84.95%   -0.37%     
==========================================
  Files         875      875              
  Lines      142521   142826     +305     
==========================================
- Hits       121589   121335     -254     
- Misses      20932    21491     +559     

[killme2008]

LGTM

[fengjiachun]

LGTM