Archive & clean up todo-python-mongo-terraform template

Archive the `Azure-Samples/todo-python-mongo-terraform` template and remove its
references from `Azure/azure-dev`. Part of the todo-* template retirement epic.

### Archive source repo
- [x] Add a deprecation notice to `Azure-Samples/todo-python-mongo-terraform` README
- [x] Archive the `Azure-Samples/todo-python-mongo-terraform` repository

### Clean up azure-dev references
- [ ] Remove the gallery entry in `cli/azd/resources/templates.json`
(`repositoryPath: todo-python-mongo-terraform`)
- [ ] Update any affected AI-agent sample-list recordings under
`cli/azd/test/functional/ai_agents/testdata/recordings/`
- [ ] Sweep docs / help / pipeline comments for `todo-python-mongo-terraform`

> Historical `CHANGELOG.md` mentions are release history — do not edit.

Part of #9094