feat: Add PostgreSQL 18 driver + driver refactor

[Unbreathable]

Description

• This MR adds a PostgreSQL driver in the new pkg/databases directory for drivers that supports PostgreSQL 18 and its new container layout
• The basic container lifecycle has been moved to a new mservices package for more easily creating drivers
• The PostgreSQL legacy driver has been upgraded to use the new API (will be maintained until PostgreSQL 20 is out)
• The real-project example has been updated to use the new PostgreSQL 18 driver

[Copilot]

Pull request overview

Adds a new PostgreSQL 18 service driver under pkg/databases/postgres, refactors Docker container lifecycle logic into reusable mrunner/services helpers, and migrates the legacy driver + real-project example to the new API.

Changes:

• Introduce new PostgreSQL 18 driver (pkg/databases/postgres) with container lifecycle, health checks, initialization, and instruction handling.
• Add mrunner/services helpers for managed container create/recreate + container exec, and update the legacy driver to use them.
• Update examples/real-project to use the new PostgreSQL 18 driver.

Reviewed changes

Copilot reviewed 12 out of 13 changed files in this pull request and generated 13 comments.

Show a summary per file

File	Description
pkg/databases/postgres/postgres.go	New PostgreSQL 18 driver implementation and config helpers
pkg/databases/postgres/postgres_container.go	Container creation/health/init for new driver via mservices
pkg/databases/postgres/postgres_instruct.go	Clear/drop table instructions for new driver
pkg/databases/postgres/go.mod	New submodule definition for postgres driver
pkg/databases/postgres/go.sum	Dependency locks for new postgres submodule
pkg/databases/postgres-legacy/postgres_legacy_container.go	Refactored legacy container lifecycle to use mservices helpers
pkg/databases/postgres-legacy/postgres_legacy.go	Updated legacy driver docs/comments
pkg/databases/postgres-legacy/go.mod	Dependency cleanup / reclassification (direct -> indirect)
mrunner/services/containers.go	New shared “managed container” creation/recreate logic
mrunner/services/containers_exec.go	New shared helper to exec commands in containers
go.work	Add postgres driver submodule to workspace
examples/real-project/starter/config.go	Switch example to new postgres (18) driver
examples/real-project/go.mod	Switch example module dependency/replace to new postgres driver

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