fix(cdc): fix some cdc backfill issues

[StrikeW]

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What's changed and what's your intention?

• drop the database connection after backfill is done
• fix the distribution of StreamCdcTableScan to SomeShard, otherwise the mview fragment will be limited to singleton but actually mv needs to be sharded.
• fix MySQL cdc table sql in rw_tables syntax error #13354

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.

[kwannoel]

#13354 (comment)

Let's add a parser test as well?

[codecov]

Codecov Report

Attention: 30 lines in your changes are missing coverage. Please review.

Comparison is base (eabfeba) 67.79% compared to head (09bddac) 67.86%.
Report is 20 commits behind head on main.

Files	Patch %	Lines
src/connector/src/source/external.rs	0.00%	19 Missing ⚠️
src/frontend/src/handler/create_source.rs	65.00%	7 Missing ⚠️
src/stream/src/from_proto/stream_scan.rs	0.00%	3 Missing ⚠️
src/stream/src/from_proto/source/trad_source.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13397      +/-   ##
==========================================
+ Coverage   67.79%   67.86%   +0.06%     
==========================================
  Files        1526     1526              
  Lines      259801   259804       +3     
==========================================
+ Hits       176145   176304     +159     
+ Misses      83656    83500     -156     

Flag	Coverage Δ
rust	67.86% <34.78%> (+0.06%)	⬆️

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