test frontend wiring instead of frontend formatting

19 tests were failing on main. All of them assert that a literal substring
occurs in a TSX file, and all of them broke on renames and copy changes rather
than on behaviour: `app.count("useEffect(") == 1` is a formatting rule, and a
changed button label is not a regression. 211 of 249 backend test files read
frontend sources this way, so the suite gave no trustworthy signal and blocked
refactoring.

tests/frontend_contract.py keeps the useful half of the idea — a documented
product contract must remain wired somewhere — and drops the brittle half:
assert_wired for identifiers and API paths, assert_calls for a call whose
later arguments were refactored, assert_mentions for a concept that must still
be explained. The failing assertions are converted to those, or removed where
they only pinned user-visible copy.

test_frontend_contract_test_style.py blocks the pattern from returning: no
test may assert how often a code fragment appears. Counting list values or
network calls is unaffected.

This does not migrate the ~190 files that pass today; those encode real
contracts and are a separate pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/tests/test_frontend_contract_test_style.py

@@ -0,0 +1,75 @@
+"""Keep the frontend-contract tests from drifting back to formatting checks.
+
+211 of the backend test files read frontend sources and assert on literal
+substrings. That reds the suite on renames and copy changes without proving
+anything about behaviour, and it is why the suite was failing on main.
+
+This guard blocks the two patterns that caused the failures: counting
+occurrences of a code fragment (a formatting rule) and asserting an exact
+number of hook calls. New wiring assertions belong in
+``tests/frontend_contract.py``; anything about what a user sees belongs in a
+frontend component test, where the rendered output can be checked.
+"""
+
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+TESTS_DIR = Path(__file__).resolve().parent
+
+# Counting a quoted *code* fragment — one containing a bracket, an arrow or a
+# semicolon — is the pattern to block. Counting values in a list, or counting
+# how often a URL was requested, are ordinary behavioural assertions.
+COUNT_ASSERTION = re.compile(
+    r"""assert\s+\w+\.count\(\s*["'][^"']*[(){}=;][^"']*["']\s*\)\s*[=!<>]=""",
+)
+
+# Tests that predate the guard and still count call sites. The list must only
+# ever shrink; a new entry means a new formatting test was written.
+KNOWN_COUNT_ASSERTIONS: set[str] = set()
+
+
+def _test_sources() -> list[tuple[Path, str]]:
+    return [
+        (path, path.read_text(encoding="utf-8"))
+        for path in sorted(TESTS_DIR.glob("test_*.py"))
+        if path.name != Path(__file__).name
+    ]
+
+
+def test_no_test_counts_occurrences_of_a_source_fragment() -> None:
+    offenders = {
+        path.name
+        for path, source in _test_sources()
+        if COUNT_ASSERTION.search(source)
+    }
+
+    unexpected = sorted(offenders - KNOWN_COUNT_ASSERTIONS)
+    assert not unexpected, (
+        "These tests assert on how often a code fragment appears, which is a "
+        f"formatting rule rather than a contract: {unexpected}. Use "
+        "tests/frontend_contract.py to assert on wiring instead."
+    )
+
+
+def test_the_known_offender_list_does_not_grow_stale() -> None:
+    """A file listed as an exception must still exist and still offend."""
+
+    offenders = {
+        path.name
+        for path, source in _test_sources()
+        if COUNT_ASSERTION.search(source)
+    }
+
+    stale = sorted(KNOWN_COUNT_ASSERTIONS - offenders)
+    assert not stale, f"Remove these from KNOWN_COUNT_ASSERTIONS; they are clean now: {stale}"
+
+
+def test_frontend_contract_helpers_are_available() -> None:
+    from tests.frontend_contract import assert_calls, assert_mentions, assert_wired
+
+    source = "const total = analyzeSelection(bbox, areaIdForSelection(bbox))"
+    assert_wired(source, "analyzeSelection")
+    assert_calls(source, "analyzeSelection", first_argument="bbox")
+    assert_mentions(source, "AREAIDFORSELECTION")