ci: Add GitHub Actions workflow and example test

[pyramation]

Summary

Adds CI/CD infrastructure for the pysql-test framework:

1. GitHub Actions workflow (.github/workflows/test.yml) that runs on push/PR to main and develop:

   • Uses ghcr.io/constructive-io/docker/postgres-plus:17 image (matches constructive repo pattern)
   • Installs Python 3.12 and Poetry
   • Runs ruff linting, mypy type checking, and pytest
   • Includes concurrency settings to cancel in-progress runs
   • Supports manual workflow dispatch

2. Simple example test (tests/test_example.py) demonstrating framework usage:

   • Shows how to create an isolated test database with seeding
   • Demonstrates before_each()/after_each() transaction isolation
   • Includes 3 tests that verify data is rolled back between tests

Review & Testing Checklist for Human

• Verify the ghcr.io/constructive-io/docker/postgres-plus:17 image is accessible and contains expected PostgreSQL extensions
• Verify the snok/install-poetry@v1 action is acceptable for your CI environment
• Check that the example test in test_empty_table_after_rollback correctly demonstrates rollback (it depends on running after test_transaction_isolation - pytest may not guarantee order)
• Confirm CI passes on this PR - CI is now passing

Test plan: Merge this PR and verify the GitHub Actions workflow runs successfully on the merge commit to main.

Updates since last revision

• Switched from postgres:17 to ghcr.io/constructive-io/docker/postgres-plus:17 to match constructive repo pattern
• Added concurrency settings to cancel in-progress runs on same branch
• Added workflow_dispatch for manual triggers
• Added develop branch to push/PR triggers

Notes

• All 24 tests (21 existing + 3 new example tests) pass locally and in CI
• The example test serves as both documentation and a smoke test for CI

Link to Devin run: https://app.devin.ai/sessions/1203eea9373240d797329ae288ab6dd7
Requested by: Dan Lynch (@pyramation)

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