test(proxy): align hooks regression test with Bug 3 recount semantics

test_anthropic_hooks_do_not_break_extract_user_query_lookup mocks
pipeline.apply to return tokens_after=40 and a tiny compressed
message. The pre-Bug-3 proxy trusted the mock's tokens_after and
emitted x-headroom-tokens-after: 40. After issue #327 Bug 3 the
proxy recounts optimized_tokens from result.messages with its
own tokenizer (the mocked "compressed" string counts to 11), so the
header asserted against the wrong tokenizer's number.

Compute the expected value from the same tokenizer the proxy uses
(get_tokenizer("claude-sonnet-4-6")) and assert the recounted
header matches that. Add a tokens_before > tokens_after invariant
so the spirit of the test (compression actually reduced bytes) is
preserved without coupling to a specific tokenizer's calibration.

tests/test_proxy_hooks_regression.py

@@ -37,9 +37,18 @@ def test_anthropic_hooks_do_not_break_extract_user_query_lookup(tmp_path, monkey
         proxy = client.app.state.proxy
         large_user_content = "hello " * 200
 
+        # Mocked pipeline output. The proxy now recounts `optimized_tokens`
+        # using its own tokenizer instead of trusting `result.tokens_after`
+        # (issue #327 / Bug 3: cross-tokenizer comparison broke the
+        # inflation guard and zeroed out compression on Anthropic).
+        # The mocked content below tokenizes to a known value with the
+        # proxy's EstimatingTokenCounter; the assertion below is computed
+        # from that same tokenizer so the test stays robust to any future
+        # tokenizer recalibration.
+        compressed_messages = [{"role": "user", "content": "compressed"}]
         proxy.anthropic_pipeline.apply = MagicMock(
             return_value=SimpleNamespace(
-                messages=[{"role": "user", "content": "compressed"}],
+                messages=compressed_messages,
                 transforms_applied=["test_transform"],
                 timing={},
                 tokens_before=100,
@@ -76,6 +85,19 @@ def test_anthropic_hooks_do_not_break_extract_user_query_lookup(tmp_path, monkey
 
         assert response.status_code == 200
         assert proxy.anthropic_pipeline.apply.called
-        assert response.headers["x-headroom-tokens-after"] == "40"
-        assert int(response.headers["x-headroom-tokens-before"]) > 40
+
+        # `x-headroom-tokens-after` reflects the proxy-side recount of the
+        # pipeline's returned messages (NOT the mock's `tokens_after=40`).
+        # See issue #327 Bug 3: prior behavior trusted pipeline tokens_after
+        # which used a different tokenizer than `original_tokens`, breaking
+        # the inflation guard.
+        from headroom.tokenizers.registry import get_tokenizer
+
+        expected_after = get_tokenizer("claude-sonnet-4-6").count_messages(compressed_messages)
+        assert response.headers["x-headroom-tokens-after"] == str(expected_after)
+        tokens_before = int(response.headers["x-headroom-tokens-before"])
+        tokens_after = int(response.headers["x-headroom-tokens-after"])
+        assert tokens_before > tokens_after, (
+            f"compression should reduce tokens: before={tokens_before} after={tokens_after}"
+        )
         assert int(response.headers["x-headroom-tokens-saved"]) > 0