test: complete detection fallback-chain coverage

Implement the two remaining detection branches: python3 PATH probe and
the sys.version_info runtime fallback (subprocess stubbed via monkeypatch,
cwd isolated so a real .python-version can't shadow the branch under test).
detection.py is now fully covered — 12 tests.

Mark the gap CLOSED in TEST-STRATEGY.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ayhammouda

.github/TEST-STRATEGY.md

@@ -32,7 +32,7 @@ behavioral test and appear in the schema snapshot.
 | `list_versions`        | `test_services`, `test_multi_version`                                | unit + integration| GOOD   |
 | `compare_versions`     | `test_compare_versions` (15), `test_services`                        | unit             | GOOD   |
 | `lookup_package_docs`  | `test_package_docs` (8)                                              | unit only        | THIN   |
-| `detect_python_version`| — none —                                                             | —                | **GAP**|
+| `detect_python_version`| `test_detection` (12)                                               | unit             | GOOD   |
 
 Cross-cutting coverage:
 
@@ -76,17 +76,18 @@ packages, not `server.py` (intentionally thin) or `__main__.py`.
 
 ## 5. Known gaps
 
-1. **`detection.py` is untested (HIGH).** `detect_python_version` is 1 of 6
-   public tools and has three branches (`.python-version` file → `python3` in
-   PATH → `sys.version_info` fallback) plus `_parse_major_minor` regex parsing
-   and `match_to_indexed`. No `tests/test_detection.py` exists. Confirmed across
-   prior audits. **Closing this is the top priority below.**
+1. **`detection.py` — CLOSED (2026-05-29).** `tests/test_detection.py` now
+   covers all three branches of the fallback chain (`.python-version` file →
+   `python3` in PATH → `sys.version_info`), `_parse_major_minor` parsing, and
+   `match_to_indexed` — 12 tests. The isolation pattern (`monkeypatch.chdir` to
+   escape a real `.python-version`, `monkeypatch.setattr` on `subprocess.run`)
+   is the reference for testing order-dependent environment probing.
 2. **`lookup_package_docs` has no stdio smoke (LOW).** Covered at the service
    layer only; the PyPI-allowlist trust boundary is never exercised end-to-end.
 3. **No negative version-resolution E2E (LOW).** Unknown-version errors are
    unit-tested but not asserted over the stdio boundary.
 
-## 6. Example test cases for the top gap (`detection.py`)
+## 6. Reference cases — `detection.py` (now implemented in `test_detection.py`)
 
 | Case                                   | Expectation                              |
 |----------------------------------------|------------------------------------------|

tests/test_detection.py

@@ -16,6 +16,7 @@
 from __future__ import annotations
 
 import subprocess
+import sys
 
 import pytest
 
@@ -86,26 +87,39 @@ def fake_run(*args, **_kwargs):
     assert source == "python3 in PATH"
 
 
-# TODO(you): implement the remaining two branches of the fallback chain.
-#
-# These are the cases where the test *design* matters most — you have to
-# neutralize the branches above the one under test. Both start from an empty
-# cwd so no real .python-version interferes:
-#
-#     monkeypatch.chdir(tmp_path)   # escape any real .python-version
-#
-# 1) test_detects_from_path_probe:
-#       - Patch detection.subprocess.run to return a CompletedProcess with
-#         returncode=0 and stdout="Python 3.10.9\n".
-#       - Assert (version, source) == ("3.10", "python3 in PATH").
-#
-# 2) test_falls_back_to_runtime_when_no_python3:
-#       - Make the PATH probe fail: patch detection.subprocess.run to raise
-#         FileNotFoundError (python3 absent). Decide what the function should
-#         return — it falls back to the server's own interpreter. Assert the
-#         source is "server runtime" and the version matches
-#         f"{sys.version_info.major}.{sys.version_info.minor}".
-#
-# Why this is the meaningful part: detection is order-dependent, so a test that
-# forgets to chdir() or forgets to stub subprocess will pass on your machine
-# and fail in CI (or vice-versa). The isolation is the assertion.
+def test_detects_from_path_probe(tmp_path, monkeypatch) -> None:
+    """Branch 2: no .python-version file, so the python3 PATH probe wins.
+
+    chdir to an empty tmp dir to neutralize branch 1 (any real
+    .python-version on the host), then stub the probe deterministically.
+    """
+    monkeypatch.chdir(tmp_path)
+
+    def fake_run(*args, **_kwargs):
+        return subprocess.CompletedProcess(args, 0, stdout="Python 3.10.9\n", stderr="")
+
+    monkeypatch.setattr(detection.subprocess, "run", fake_run)
+
+    version, source = detect_python_version()
+
+    assert version == "3.10"
+    assert source == "python3 in PATH"
+
+
+def test_falls_back_to_runtime_when_no_python3(tmp_path, monkeypatch) -> None:
+    """Branch 3: no file and no python3 on PATH -> server's own interpreter.
+
+    Neutralize branch 1 (empty cwd) and force branch 2 to fail by making the
+    probe raise FileNotFoundError, exactly as a missing python3 would.
+    """
+    monkeypatch.chdir(tmp_path)
+
+    def boom(*args, **_kwargs):
+        raise FileNotFoundError("python3 not found")
+
+    monkeypatch.setattr(detection.subprocess, "run", boom)
+
+    version, source = detect_python_version()
+
+    assert source == "server runtime"
+    assert version == f"{sys.version_info.major}.{sys.version_info.minor}"