feat(agents-server): accept caller-owned migration clients

[adityavkk]

Summary

• Accept a caller-owned PgClient in runMigrations and leave its lifecycle with the caller.
• Keep the existing PostgreSQL URL form and close its internal client after success or failure.
• Cover both public API forms and each ownership path with focused tests.

Callers that pass a client must close it themselves.

Test plan

• pnpm --filter @electric-ax/agents-server test
• pnpm --filter @electric-ax/agents-server typecheck
• pnpm --filter @electric-ax/agents-server stylecheck
• pnpm --filter @electric-ax/agents-server build
• pnpm run format:check
• GITHUB_BASE_REF=main node scripts/check-changeset.mjs

Closes #4746

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