test: improve test coverage for pkg/datasource

[everfid-ever]

Which issue(s) this PR fixes:

Fixes #933

[Copilot]

Pull Request Overview

This PR adds comprehensive test coverage for the datasource initialization functionality. The Init function registers SQL drivers for Seata's AT and XA transaction modes.

• Adds test file pkg/datasource/init_test.go with 23 test functions and 4 benchmark functions
• Implements thread-safe test initialization using sync.Once to handle non-idempotent Init() function
• Tests driver registration, persistence, concurrency, and uniqueness

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.50%. Comparing base (0747d09) to head (5c09a7e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #973   +/-   ##
=======================================
  Coverage   48.49%   48.50%           
=======================================
  Files         199      200    +1     
  Lines       12329    12331    +2     
=======================================
+ Hits         5979     5981    +2     
  Misses       5880     5880           
  Partials      470      470           

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[slievrly]

@everfid-ever There are no new cover lines.