fix(migrations): resolve extension SQL query names

[cofin]

Summary

• isolate SQL query loaders for each registered extension migration directory
• preserve raw filename-local query directives while retaining extension-prefixed tracker and bulk-loading identities
• document both supported extension migration layouts and add synchronous, asynchronous, collision, and integration regressions

Root cause

Extension migration discovery prefixes versions for tracking, but SQL query names are derived from the raw migration filename. The runners checked the shared loader using the prefixed tracker version, so valid extension-directory SQL migrations appeared to have no upgrade query. Reusing one shared loader also caused collisions when multiple extensions shipped the same numeric version.

Impact

Third-party extensions can now ship ordinary SQL migrations such as 0001_create_table.sql with migrate-0001-up and migrate-0001-down. SQLSpec still records those migrations as ext_<extension>_0001, and existing main-directory prefixed migrations remain unchanged.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.24390% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.36%. Comparing base (7bf79bb) to head (ab5b7ba).

Files with missing lines	Patch %	Lines
sqlspec/migrations/runner.py	90.24%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #686      +/-   ##
==========================================
+ Coverage   77.34%   77.36%   +0.01%     
==========================================
  Files         476      476              
  Lines       68160    68193      +33     
  Branches     9376     9385       +9     
==========================================
+ Hits        52720    52758      +38     
+ Misses      12032    12026       -6     
- Partials     3408     3409       +1     

Flag	Coverage Δ
integration	60.88% <56.09%> (-0.02%)	⬇️
py3.10	75.68% <90.24%> (+0.01%)	⬆️
py3.11	75.69% <90.24%> (+11.06%)	⬆️
py3.12	75.68% <90.24%> (+<0.01%)	⬆️
py3.13	75.69% <90.24%> (+<0.01%)	⬆️
py3.14	76.57% <90.24%> (+0.02%)	⬆️
unit	64.94% <85.36%> (+0.02%)	⬆️

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Files with missing lines	Coverage Δ
sqlspec/migrations/runner.py	85.58% <90.24%> (+0.32%)	⬆️

... and 2 files with indirect coverage changes

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