test(install): harden uv-python-path regression test against future drift

Self-review follow-ups on the salvage of #22494:

W2 — Added encoding="utf-8" to read_text() calls. scripts/install.sh
contains 48 em-dash ("—") characters and ~1500 non-ASCII bytes total;
on Windows with cp1252 default locale, bare read_text() would raise
UnicodeDecodeError. Project-wide cleanup of the other 11 similar sites
across 5 install_sh test files is deferred to a separate follow-up.

W3 — Bound the branch-containment check by the function body (head
"resolve_install_layout() {" / tail "\n}\n") instead of by "next
`return 0` after the marker". scripts/install.sh has 5 additional
`return 0` statements between resolve_install_layout's first one and
EOF; if a future maintainer hoists the export above another conditional
with its own early-return or inserts an early-return between the marker
and the export, the old assertion still passes while the export is
unreachable. The body-bounded slice makes that class of regression
visible.

Also added more specific assertion messages and a guard for the body
extraction to fail loudly if the function signature ever changes.

tests/test_install_sh_root_fhs_uv_python_path.py

@@ -14,8 +14,26 @@ REPO_ROOT = Path(__file__).resolve().parent.parent
 INSTALL_SH = REPO_ROOT / "scripts" / "install.sh"
 
 
+def _resolve_install_layout_body() -> str:
+    """Return just the body of resolve_install_layout(), bounded by its
+    opening signature and the next top-level ``}`` close brace.
+
+    Using the function body (not "first ``return 0`` after a marker") guards
+    the tests below against future refactors that hoist the export above
+    another conditional with its own early-return, or that insert an early-
+    return between the marker and the export — both of which would leave the
+    export unreachable while a less-strict assertion still passed.
+    """
+    text = INSTALL_SH.read_text(encoding="utf-8")
+    head, _, rest = text.partition("resolve_install_layout() {\n")
+    assert rest, "Could not find resolve_install_layout() in scripts/install.sh"
+    body, _, _ = rest.partition("\n}\n")
+    assert body, "Could not find resolve_install_layout() closing brace"
+    return body
+
+
 def test_root_fhs_layout_exports_world_readable_uv_python_dirs() -> None:
-    text = INSTALL_SH.read_text()
+    text = INSTALL_SH.read_text(encoding="utf-8")
 
     assert 'export UV_PYTHON_INSTALL_DIR="${UV_PYTHON_INSTALL_DIR:-/usr/local/share/uv/python}"' in text
     assert 'export UV_PYTHON_BIN_DIR="${UV_PYTHON_BIN_DIR:-/usr/local/share/uv/bin}"' in text
@@ -23,13 +41,19 @@ def test_root_fhs_layout_exports_world_readable_uv_python_dirs() -> None:
 
 def test_root_fhs_uv_python_export_is_inside_root_branch() -> None:
     """The export must live in the root-FHS branch of resolve_install_layout,
-    above its `return 0`, so non-root and Termux installs are unaffected."""
-    text = INSTALL_SH.read_text()
+    after ``ROOT_FHS_LAYOUT=true`` and before the branch's ``return 0``, so
+    non-root and Termux installs are unaffected. Bound the slice by the
+    function body (not "next return 0" in the whole file) so the assertion
+    can't accept an unreachable export."""
+    body = _resolve_install_layout_body()
 
     marker = 'ROOT_FHS_LAYOUT=true'
-    assert marker in text
-    after_marker = text.split(marker, 1)[1]
+    assert marker in body
+    after_marker = body.split(marker, 1)[1]
     return_idx = after_marker.find('return 0')
     export_idx = after_marker.find('UV_PYTHON_INSTALL_DIR')
-    assert export_idx != -1
-    assert export_idx < return_idx
+    assert export_idx != -1, "UV_PYTHON_INSTALL_DIR export missing from root-FHS branch"
+    assert return_idx != -1, "root-FHS branch must end with `return 0`"
+    assert export_idx < return_idx, (
+        "Export must precede the branch's `return 0` — otherwise unreachable"
+    )