feat(terraform.test): add composite action for Terraform tests

[posquit0]

👋 Background

Terraform integration has reusable format, validation, and lint actions, but no standalone action for executing .tftest.hcl and .tftest.json tests.

⚙️ Description

Add .github/actions/terraform.test following the existing Terraform composite action conventions: caller-managed CLI installation, snake_case inputs, shell.run outputs, failure annotations, and github.step-summary failure details.

• Expose target_dir, workspace, test_directory, filters, var_files, vars, format, verbose, parallelism, junit_xml, cloud_run, and annotations_enabled.
• Preserve each line of repeated options as a single argument, including spaces and HCL expressions. Load variable files before explicit variable assignments.
• Initialize local runs without a backend and pass the selected test directory to both init and test, so modules referenced by custom test directories are installed. Skip local init for HCP Terraform execution.
• Return skipped, stdout, stderr, and exitcode; report init failures without running tests. Read captured diagnostics for annotations without executing tests again.
• Document defaults, path semantics, version requirements, and report handling. Existing integration workflows are unchanged.

Validation:

• Terraform 1.16.5: all 12 regression tests passed against temporary provider-free fixtures, including failure summaries, JSON diagnostics, repeated options, JUnit success/failure reports, and custom test modules.
• Terraform 1.6.2: 10 passed; 2 skipped for newer CLI features.
• YAML lint, GitHub Actions metadata schema validation, Bash syntax checks, ShellCheck for all action scripts, and git diff --check passed.

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🚨 To Reviewers

The default invocation supports Terraform 1.6+. JUnit requires 1.11+; parallelism and initialization of test-only configurations require 1.12+. junit_xml and cloud_run are mutually exclusive. Remote argument forwarding is covered by a stub; authenticated HCP Terraform execution was not exercised.