feat(graphile-test): add withTransaction to pgClient in test context

[pyramation]

Summary

Adds a savepoint-based withTransaction implementation to the pgClient provided inside the withPgClient callback in graphile-test's execution context.

In production, @dataplan/pg provides withTransaction on the pgClient. In tests, the raw pg.Client from pgsql-test lacks this method, causing any Graphile plugin that calls pgClient.withTransaction() (e.g. graphile-presigned-url-plugin) to crash at runtime unless each test file manually monkey-patches the method.

This fix augments the client lazily (guarded by typeof check) with a savepoint-based implementation that nests cleanly inside the test harness's outer transaction. Downstream tests in constructive-db can now remove their per-file monkey patches.

Review & Testing Checklist for Human

• Verify the savepoint-based withTransaction behaves correctly when the callback throws (should ROLLBACK TO SAVEPOINT and re-throw, not leave the transaction in a broken state)
• Confirm that mutating pgClient (attaching withTransaction permanently) doesn't cause issues across multiple withPgClient calls in the same test — the if guard prevents re-assignment but the method stays attached
• After publishing, verify downstream: remove the monkey patch from constructive-db's minio-multi-scope-graphql.integration.test.ts and confirm the storage upload tests still pass

Notes

• No unit test added in this PR. The method is exercised by the presigned-url plugin's upload flow in constructive-db's integration tests. A focused unit test could be added as a follow-up.
• The eslint-disable for @typescript-eslint/no-explicit-any is required because the raw pg.Client type doesn't declare withTransaction.

Link to Devin session: https://app.devin.ai/sessions/7903d2a3e7a34c6daa605e12d6b80d9e
Requested by: @pyramation

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