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.

Validation

• make install
• make lint
• make test (7960 passed, 256 skipped)
• uv run pytest tests/unit/adapters/test_bigquery/test_migration_schema.py tests/unit/adapters/test_oracledb/test_migration_schema.py -q (8 passed)
• env SQLSPEC_ENABLE_BIGQUERY_TESTS=1 uv run pytest tests/integration/adapters/bigquery/test_migrations.py -q --run-bigquery-tests (1 passed, 3 skipped on emulator)
• make coverage was attempted after make test; it produced broad integration failures and then hung during teardown, so I terminated it without a final coverage report.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.27609% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.02%. Comparing base (6f91042) to head (b9d3ad2).

Files with missing lines	Patch %	Lines
sqlspec/adapters/bigquery/driver.py	72.72%	7 Missing and 2 partials ⚠️
sqlspec/migrations/runner.py	77.77%	3 Missing and 3 partials ⚠️
sqlspec/adapters/bigquery/core.py	96.42%	0 Missing and 1 partial ⚠️
sqlspec/adapters/oracledb/migrations.py	93.75%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #471      +/-   ##
==========================================
+ Coverage   70.78%   71.02%   +0.24%     
==========================================
  Files         438      438              
  Lines       52541    52829     +288     
  Branches     7370     7408      +38     
==========================================
+ Hits        37189    37524     +335     
+ Misses      12682    12656      -26     
+ Partials     2670     2649      -21     

Flag	Coverage Δ
integration	56.74% <75.75%> (+0.14%)	⬆️
py3.10	68.10% <93.93%> (+0.25%)	⬆️
py3.11	68.12% <93.93%> (+0.25%)	⬆️
py3.12	68.12% <93.93%> (+0.25%)	⬆️
py3.13	68.11% <93.93%> (+0.24%)	⬆️
py3.14	70.20% <93.85%> (+0.24%)	⬆️
unit	57.89% <90.23%> (+0.44%)	⬆️

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