ci: add Terraform mock unit tests for all templates for #71 (Phase 2)

[rfay]

The Issue

Part of #71 (automated testing). Phase 2: Terraform mock unit tests that run with no live infrastructure.

How This PR Solves The Issue

Adds a tests/validate.tftest.hcl file to each of the three templates using Terraform 1.7+ mock providers. The tests run entirely locally — no Coder server, no Docker, no credentials needed.

Tests per template:

Test	user-defined-web	drupal-core	freeform
Plan succeeds with defaults	✓	✓	✓
docker_container created when start_count=1	✓	✓	✓
cpu below minimum (0) rejected	✓	✓	✓
cpu above maximum (33) rejected	✓	✓	✓
memory below minimum (1 GB) rejected	✓	✓	✓
memory above maximum (129 GB) rejected	✓	✓	✓
enable_adminer off by default			✓
enable_adminer creates coder_app			✓

One non-obvious mock requirement: vscode_extensions.value is passed through jsondecode() in locals across all three templates, so the coder_parameter mock overrides the default random string with "[]" (a valid empty JSON array).

drupal-core requires cache_path in every run block since it has no default (tracked in #99 for removal).

Also adds terraform test step to the existing validate workflow matrix job and make test-templates for local use.

Manual Testing Instructions

# Run all template tests locally (requires terraform in PATH)
make test-templates

# Or per-template
cd user-defined-web && terraform test
cd drupal-core && terraform test
cd freeform && terraform test

Automated Testing Overview

This PR extends the workflow added in #100. The terraform test step runs after terraform validate in the same matrix job for each template. CI will run these 20 tests on every push and PR.

Release/Deployment Notes

No impact on deployed templates or workspaces. CI-only change.