ref(tox): Allow overriding TESTPATH via env var for targeted test runs (#5699)

### Description

Allows better agentic testing flow.

Rename internal `TESTPATH` to `_TESTPATH` in setenv and add `TESTPATH`
to `passenv`.
The pytest command now uses `{env:TESTPATH:{env:_TESTPATH}}`, so setting
TESTPATH externally overrides the default test path. This allows running
specific test files without the --override workaround:

```
  TESTPATH=tests/integrations/logging/test_logging.py tox -e py3.14-common
```

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: sl0thentr0py

.claude/settings.json

@@ -27,7 +27,13 @@
       "WebFetch(domain:docs.sentry.io)",
       "WebFetch(domain:develop.sentry.dev)",
       "Bash(grep:*)",
-      "Bash(mv:*)"
+      "Bash(mv:*)",
+      "Bash(source .venv/bin/activate)",
+      "Bash(tox:*)",
+      "Bash(tox.venv/bin/tox:*)",
+      "Bash(.tox/*/bin/python:*)",
+      "Bash(.tox/*/bin/pytest:*)",
+      "Bash(.tox/*/bin/ruff:*)"
     ],
     "deny": []
   }

.gitignore

@@ -22,6 +22,7 @@ sentry-python-serverless*.zip
 venv
 .venv
 tox.venv
+toxgen.venv
 .vscode/tags
 .pytest_cache
 .hypothesis

AGENTS.md

@@ -11,6 +11,8 @@ Use **tox** for testing (not pytest directly):
 - Test matrix configuration is in `tox.ini`
 - Integration tests: `tox -e py3.14-{integration}-v{version}`
 - Common tests: `tox -e py3.14-common`
+- Run specific test file: `TESTPATH=tests/integrations/logging/test_logging.py tox -e py3.14-common`
+- Run single test: `TESTPATH=tests/path/to/test_file.py tox -e py3.14-common -- -k "test_name"`
 
 ## Type Checking
 

scripts/populate_tox/package_dependencies.jsonl

@@ -168,29 +168,30 @@ setenv =
     gevent: PYTEST_ADDOPTS="--ignore=tests/test_shadowed_module.py"
 
     # TESTPATH definitions for test suites not managed by toxgen
-    common: TESTPATH=tests
-    gevent: TESTPATH=tests
-    integration_deactivation: TESTPATH=tests/test_ai_integration_deactivation.py
-    shadowed_module: TESTPATH=tests/test_shadowed_module.py
-    asgi: TESTPATH=tests/integrations/asgi
-    aws_lambda: TESTPATH=tests/integrations/aws_lambda
-    cloud_resource_context: TESTPATH=tests/integrations/cloud_resource_context
-    gcp: TESTPATH=tests/integrations/gcp
-    opentelemetry: TESTPATH=tests/integrations/opentelemetry
-    otlp: TESTPATH=tests/integrations/otlp
-    potel: TESTPATH=tests/integrations/opentelemetry
-    socket: TESTPATH=tests/integrations/socket
+    common: _TESTPATH=tests
+    gevent: _TESTPATH=tests
+    integration_deactivation: _TESTPATH=tests/test_ai_integration_deactivation.py
+    shadowed_module: _TESTPATH=tests/test_shadowed_module.py
+    asgi: _TESTPATH=tests/integrations/asgi
+    aws_lambda: _TESTPATH=tests/integrations/aws_lambda
+    cloud_resource_context: _TESTPATH=tests/integrations/cloud_resource_context
+    gcp: _TESTPATH=tests/integrations/gcp
+    opentelemetry: _TESTPATH=tests/integrations/opentelemetry
+    otlp: _TESTPATH=tests/integrations/otlp
+    potel: _TESTPATH=tests/integrations/opentelemetry
+    socket: _TESTPATH=tests/integrations/socket
 
     # These TESTPATH definitions are auto-generated by toxgen
     {% for integration, testpath in testpaths %}
-    {{ integration }}: TESTPATH={{ testpath }}
+    {{ integration }}: _TESTPATH={{ testpath }}
     {% endfor %}
 
 passenv =
     SENTRY_PYTHON_TEST_POSTGRES_HOST
     SENTRY_PYTHON_TEST_POSTGRES_USER
     SENTRY_PYTHON_TEST_POSTGRES_PASSWORD
     SENTRY_PYTHON_TEST_POSTGRES_NAME
+    TESTPATH
 
 usedevelop = True
 
@@ -228,7 +229,7 @@ commands =
     ; Running `pytest` as an executable suffers from an import error
     ; when loading tests in scenarios. In particular, django fails to
     ; load the settings from the test module.
-    python -m pytest -W error::pytest.PytestUnraisableExceptionWarning {env:TESTPATH} -o junit_suite_name={envname} {posargs}
+    python -m pytest -W error::pytest.PytestUnraisableExceptionWarning {env:TESTPATH:{env:_TESTPATH}} -o junit_suite_name={envname} {posargs}
 
 [testenv:linters]
 commands =