fix(tests): repair three main-branch test failures (#2306)

## Description

`main` CI is red on three independent test failures. All three are
**test-side** bugs (stale cache, semantic merge conflict, stale mock) —
no product code regressed. Each test passed in isolation but failed on
`main`, and each also blocks the `chore: release main` PR (#1923).

Closes #

## Type of Change

- [x] Bug fix (non-breaking change that fixes an issue)

## Changes Made

- **`test_l2_appends_transform_label`** — `tool_desc_max_chars()`
memoises into a module global. An earlier test in shard 1 reads it with
the env unset, pinning the cache to `0`, so this test's
`setenv("HEADROOM_TOOL_DESC_MAX_CHARS=20")` was swallowed (`assert 0 ==
20`). Reset the cache before reading and after, mirroring the sibling
`test_l2_skips_label_when_disabled`.
- **`test_dashboard_uses_cached_stats_and_lazy_history_feed_polling`** —
semantic merge conflict: #2198 (persist lifetime metrics) intentionally
retired the session-card `Filtered (lifetime)` row and moved
CLI-filtering lifetime into the history tab as `Lifetime Saved`, while
the assertion from #1433 still checked the old string. Assert the
current `Lifetime Saved` label.
- **`test_smart_crusher_log_fallback_runs_for_valid_json`** — stale
mock: #1857 made token counting whitespace-aware, so the router now
rates the JSON above the naive `len(content.split())==8` the no-op
kompress mock reported, making it look like a saving and
short-circuiting before the Log fallback. Mock now reports
`_estimate_tokens(content)` to match the router.

## Testing

- [x] Unit tests pass (`pytest`)
- [x] Linting passes (`ruff check .`)
- [x] Type checking passes (`mypy headroom`)

### Test Output

```text
$ pytest tests/test_anthropic_compaction_transforms.py \
         tests/test_proxy_dashboard_stats_cache.py \
         tests/test_transforms_content_router.py -q
78 passed, 1 skipped in 12.14s

$ ruff check <the three files>
All checks passed!
$ ruff format --check <the three files>
3 files already formatted
```

## Real Behavior Proof

- Environment: local `.venv`, Python 3.12.6, pytest 9.0.2 (same three
tests that fail on the `main` CI shards 1/3/4).
- Exact command / steps: ran the three previously-failing tests by node
id — all pass. Reproduced the shard-isolation failure for #1 by calling
`tool_desc_max_chars()` with the env unset (cache → 0) before the test,
confirmed the reset makes it pass.
- Observed result: 3/3 target tests pass; 78 passed / 1 skipped across
the three full files.
- Not tested: full suite (unchanged product code); CI shards will re-run
on this PR.

## Review Readiness

- [x] I have performed a self-review
- [x] This PR is ready for human review

## Additional Notes

`mypy headroom` (the CI-enforced scope) is unaffected — these edits
touch only `tests/`, which CI does not type-check. Once this lands on
`main`, the `chore: release main` PR (#1923) drops to just the
`test_root_server_json_matches_builder` failure, which is the release
version-bump `server.json` regen (not a code bug).

tests/test_anthropic_compaction_transforms.py

@@ -114,13 +114,18 @@ class TestAnthropicToolDescCompactionTransforms:
     ``anthropic:tool_desc_compaction`` must appear in ``transforms_applied``."""
 
     def test_l2_appends_transform_label(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        import headroom.proxy.tool_schema_compaction as _mod
         from headroom.proxy.tool_schema_compaction import (
             compact_tool_descriptions,
             tool_desc_max_chars,
         )
 
-        # Opt-in with a very short max so truncation triggers.
+        # Opt-in with a very short max so truncation triggers. Reset the
+        # per-process cache first: an earlier test in the shard may have read
+        # the (unset) env and pinned max_chars to 0, which would swallow our
+        # setenv below.
         monkeypatch.setenv("HEADROOM_TOOL_DESC_MAX_CHARS", "20")
+        _mod._TOOL_DESC_MAX_CHARS = None
 
         payload = _make_anthropic_payload_with_tools()
         max_chars = tool_desc_max_chars()
@@ -129,6 +134,8 @@ class TestAnthropicToolDescCompactionTransforms:
         body, modified, before, after = compact_tool_descriptions(payload, max_chars)
         assert modified is True
         assert before > after
+        # Don't leak the cached 20 into later tests in this shard.
+        _mod._TOOL_DESC_MAX_CHARS = None
 
     def test_l2_skips_label_when_disabled(self) -> None:
         import headroom.proxy.tool_schema_compaction as _mod

tests/test_proxy_dashboard_stats_cache.py

@@ -740,7 +740,9 @@ def test_dashboard_uses_cached_stats_and_lazy_history_feed_polling() -> None:
     assert "Lean-ctx" in html
     assert "Context Tool" in html
     assert "cliFilteringLabel + ' Filtered (this session)'" in html
-    assert "cliFilteringLabel + ' Filtered (lifetime)'" in html
+    # Lifetime CLI-filtering savings moved from the session card to the
+    # history tab as "Lifetime Saved" in #2198 (persist lifetime metrics).
+    assert "cliFilteringLabel) + ' Lifetime Saved'" in html
 
 
 def test_dashboard_session_metrics_do_not_repeat_proxy_tokens_without_new_context() -> None:

tests/test_transforms_content_router.py

@@ -693,11 +693,14 @@ def test_smart_crusher_log_fallback_runs_for_valid_json(
 
     monkeypatch.setattr(router, "_get_smart_crusher", lambda: NoopSmartCrusher())
     monkeypatch.setattr(router, "_get_log_compressor", lambda: ShrinkingLogCompressor())
-    # Kompress no-op → Log fallback fires.
+    # Kompress no-op → Log fallback fires. A faithful no-op reports the same
+    # token count the router computed for the (unchanged) content — using
+    # _estimate_tokens, not a naive word split, so it isn't mistaken for a
+    # saving once #1857's whitespace-aware counting rates the JSON above 8.
     monkeypatch.setattr(
         router,
         "_try_ml_compressor",
-        lambda content, context, question=None: (content, len(content.split())),
+        lambda content, context, question=None: (content, _estimate_tokens(content)),
     )
 
     compressed, _compressed_tokens, strategy_chain = router._apply_strategy_to_content(