feat(migrations): support specifying schema for migrations

[cofin]

Summary

• Add migration default_schema and tracker schema resolution through migration config, commands, runners, and utilities.
• Keep BigQuery on the shared SyncMigrationTracker; remove the custom adapter tracker path.
• Patch BigQuery SQLGlot DDL rendering so shared tracker SQL emits PRIMARY KEY NOT ENFORCED and orders DEFAULT before NOT NULL.
• Simplify Oracle migration schema handling to uppercase unquoted schema names and use shared identifier quoting.
• Gate BigQuery full migration workflow integration tests behind native BigQuery because the emulator rejects column DEFAULT values in tracker DDL.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 97.05015% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.06%. Comparing base (6f91042) to head (be49264).

Files with missing lines	Patch %	Lines
sqlspec/migrations/runner.py	90.47%	1 Missing and 5 partials ⚠️
sqlspec/adapters/adbc/driver.py	89.65%	2 Missing and 1 partial ⚠️
sqlspec/adapters/oracledb/migrations.py	95.45%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #471      +/-   ##
==========================================
+ Coverage   70.78%   71.06%   +0.28%     
==========================================
  Files         438      438              
  Lines       52541    52841     +300     
  Branches     7370     7407      +37     
==========================================
+ Hits        37189    37553     +364     
+ Misses      12682    12652      -30     
+ Partials     2670     2636      -34     

Flag	Coverage Δ
integration	56.76% <78.46%> (+0.17%)	⬆️
py3.10	68.13% <94.69%> (+0.27%)	⬆️
py3.11	68.15% <94.69%> (+0.27%)	⬆️
py3.12	68.14% <94.69%> (+0.27%)	⬆️
py3.13	68.14% <94.69%> (+0.27%)	⬆️
py3.14	70.22% <94.61%> (+0.26%)	⬆️
unit	57.87% <87.31%> (+0.42%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.