fix(migrations): eliminate duplicate SQL file loading

[cofin]

Fixes #118

Summary

Removes unnecessary clear_cache() calls that were causing SQL files to be loaded and parsed 3 times during a single migration operation.

Root Cause

SQLFileLoader.clear_cache() was being called before every migration operation, destroying CoreSQLFileLoader's internal cache (_files, _queries, _query_to_file). This forced files to be re-parsed multiple times during a single migration run.

Evidence from user logs:

INFO - sqlspec.loader - loaders - Loading SQL files
INFO - sqlspec.loader - loaders - Loaded 1 SQL files with 2 new queries in 0.883ms
INFO - sqlspec.loader - thread - Loading SQL files
INFO - sqlspec.loader - thread - Loaded 1 SQL files with 2 new queries in 2.596ms
INFO - sqlspec.loader - loaders - Loading SQL files
INFO - sqlspec.loader - loaders - Loaded 1 SQL files with 2 new queries in 0.216ms

Changes

• Removed clear_cache() from SQLFileLoader.get_up_sql()
• Removed clear_cache() from SQLFileLoader.get_down_sql()
• Removed clear_cache() from SQLFileLoader.validate_migration_file()
• Removed clear_cache() from SyncMigrationRunner.load_migration()
• Removed clear_cache() from AsyncMigrationRunner.load_migration()

Total: 5 lines deleted

Why This Works

CoreSQLFileLoader already has proper caching built-in (see sqlspec/loader.py:428-429):

if path_str in self._files:
    return  # Early exit if cached

The cache clearing was unnecessary and counterproductive.

Impact

• Files now loaded once per migration operation (down from 3x)
• ~66% reduction in file I/O and parsing overhead
• User will see single "Loading SQL files" log entry instead of 3

Testing

• Added test_sql_loader_caches_files to verify cache persistence across operations
• All 126 tests passing (90 unit + 36 integration)
• No functional changes or breaking changes
• Verified with both sync and async migration runners

Breaking Changes

None - internal optimization only.