Adding 5K tables integration test for schema migration

[sm745052]

Adds a 5,000-table integration test for MySQL direct connect and introduces a BatchedInfoSchema interface to allow bulk metadata fetching, improving performance for large schemas.

Changes

• BatchedInfoSchema: New interface in infoschema.go for bulk fetching of columns, constraints, FKs, and indexes.
• Parallel Batching: Updates GenerateSrcSchema to process tables in parallel batches.
• 5k Test: Adds TestIntegration_MYSQL_DirectConnect_5000Tables and 5,000 table SQL dump.
• Corrected TestGetConstraints_CheckConstraintsTableAbsent: Updated the test to expect success when the CHECK_CONSTRAINTS table is missing. Now validates the fallback logic that uses a simpler query to fetch TABLE_CONSTRAINTS when the check constraints table is unavailable. Previously, the test was incorrectly asserting failure due to an incomplete mock setup for the fallback query.
• Ignored ExpressionId in Source Schema Assertions: Modified the test helper assertSrcColDef in internal/test_util.go to ignore ExpressionId for both expected and actual columns. This avoids test brittleness caused by non-deterministic ID generation order during parallel batch processing.

Batch Size Experiments (5k Tables)

Batch Size	Result	Time
100	PASS	~57s
50	PASS	~94s
10	PASS	~378s
1	TIMEOUT	>30m

Conclusion: Larger batch sizes significantly reduce schema processing time. The PR currently defaults to 50.

[codecov]

Codecov Report

❌ Patch coverage is 70.21944% with 95 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.19%. Comparing base (03e1b89) to head (7bffe29).

Files with missing lines	Patch %	Lines
sources/common/infoschema.go	0.00%	59 Missing ⚠️
sources/mysql/infoschema.go	87.50%	21 Missing and 11 partials ⚠️
internal/test_util.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1324      +/-   ##
==========================================
+ Coverage   51.15%   51.19%   +0.03%     
==========================================
  Files         202      202              
  Lines       22639    22796     +157     
  Branches      552      552              
==========================================
+ Hits        11582    11671      +89     
- Misses      10375    10437      +62     
- Partials      682      688       +6     

Components	Coverage Δ
backend-apis	46.10% <ø> (ø)
backend-library	56.84% <70.21%> (-0.01%)	⬇️
cli	26.65% <ø> (ø)
frontend	39.34% <ø> (ø)

Files with missing lines	Coverage Δ
internal/test_util.go	28.74% <0.00%> (-0.71%)	⬇️
sources/mysql/infoschema.go	79.43% <87.50%> (+1.99%)	⬆️
sources/common/infoschema.go	7.77% <0.00%> (-3.00%)	⬇️

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