fix: write Terragrunt plan artifacts to the run's unique temp dir

[jcoffi]

Fixes #204

Sibling of #202 / #203, which fix the same temp-path collision for plain Terraform in tf_initplan. This PR applies the same fix to the Terragrunt path.

Type of Change

• Bug Fix
• New Feature
• Refactor
• Documentation

What this changes

tg_initplan() in modules/tgwrapper.py wrote its plan artifacts (tg_tfplan.bin, tg_tfplan.json, tg_tfgraph.dot) into tempfile.gettempdir(), the shared temp root (usually /tmp), under fixed names. Two terravision runs against Terragrunt sources at once then write to the same files and overwrite each other's plan and graph.

tg_run_all_plan() (multi-module) runs tg_initplan() once per child module, so it shares the same paths.

The fix writes the artifacts into tfwrapper.temp_dir.name, the per-process TemporaryDirectory terravision already uses for Terraform artifacts. tempfile is no longer referenced in the module, so the import is removed.

-    tfplan_path = os.path.join(tempfile.gettempdir(), "tg_tfplan.bin")
-    tfplan_json_path = os.path.join(tempfile.gettempdir(), "tg_tfplan.json")
-    tfgraph_path = os.path.join(tempfile.gettempdir(), "tg_tfgraph.dot")
+    tfplan_path = os.path.join(tfwrapper.temp_dir.name, "tg_tfplan.bin")
+    tfplan_json_path = os.path.join(tfwrapper.temp_dir.name, "tg_tfplan.json")
+    tfgraph_path = os.path.join(tfwrapper.temp_dir.name, "tg_tfgraph.dot")

Why this is safe

• tfwrapper.temp_dir is the module-level TemporaryDirectory created at import, already used for Terraform plan artifacts and as TF_DATA_DIR. tgwrapper already depends on tfwrapper for _write_override, convert_dot_to_json, and tf_makegraph, so this adds no new coupling.
• The filenames keep their tg_ prefix, so they do not clash with the Terraform artifacts in the same directory.
• Within a single run the paths are reused sequentially (each module's plan is decoded before the next module runs), exactly as before. Only the directory changed, from shared to per-process.
• _run_terragrunt_plan() and _decode_tg_plan() receive the same path variables, so they follow the new location automatically.

Testing

• python -m black --check modules/tgwrapper.py passes.
• python -m py_compile modules/tgwrapper.py passes.
• The Terragrunt integration tests in tests/test_tgwrapper.py are marked @pytest.mark.slow and exercise tg_initplan / tg_run_all_plan end to end. They assert on the returned tfdata (resource types, codepath, override cleanup), not on the temp-file location, so this change does not affect them. I could not run the slow suite in my environment (no terragrunt binary available).
• No new regression test added, matching the minimal approach taken for the Terraform counterpart in fix: write Terraform plan artifacts to the run's unique temp dir #203. Happy to add one together with a small refactor if you would prefer.

A note on scope

This is intentionally a single focused change for the Terragrunt path, paired with the Terraform fix in #203. Between them they cover both tf_initplan and tg_initplan.

Checklist

All Submissions:

• Checked for other open PRs covering the same change
• Written Documentation/Tests (see the Testing note above)
• Done my own code review
• Disclosed AI tool/model use (below)

AI Assistance Declaration

• Tools used: opencode (CLI coding agent)
• Model: Claude Opus 4 (anthropic/claude-opus-4-8)
• Scope: identified the shared-path collision in tg_initplan, applied the fix, removed the now-unused tempfile import, and wrote this PR.

Checklist for Changes to Core Features:

• Minor bugfix, raised directly as the template allows for minor fixes
• PR is focused on a single change
• Explanation of what and why is included above
• New tests not added (rationale in Testing)
• Verified the existing slow Terragrunt tests do not assert on the changed path

[patrickchugh]

@jcoffi Thanks for the PR.