session-memory: denoise error fingerprints (WP-0006 follow-up)

Tighten _is_failed: exclude successful hub JSON responses (top-level no-error
payloads) and file-read snapshots (numbered cat -n source lines) that were
polluting error_snippets. JSON verdict classifies error vs success payloads
directly. Cuts distinct fingerprints 444 -> 269 (~40%) over the real corpus with
the top errors unchanged. Assessment caveat updated. 5 new tests; suite 102/102.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_digest_errors.py

@@ -59,6 +59,33 @@ def test_clean_tool_result_not_mined():
     assert _error_snippets(events, blobs) == []
 
 
+def test_success_json_not_mined():
+    # a hub MCP success payload mentioning 'error' deep inside is NOT a failure
+    blobs = {"b1": '{"result": "{\\"domain\\": \\"custodian\\", \\"note\\": \\"no errors\\"}"}'}
+    events = [_ev(0, "tool_result", tool="mcp__state-hub__get_domain_summary", payload_ref="b1")]
+    assert _error_snippets(events, blobs) == []
+
+
+def test_error_json_still_mined():
+    blobs = {"b1": '{"detail": "Invalid request parameters"}'}
+    events = [_ev(0, "tool_result", tool="Bash", payload_ref="b1")]
+    snips = _error_snippets(events, blobs)
+    assert len(snips) == 1
+
+
+def test_plain_mcp_error_still_mined():
+    blobs = {"b1": "MCP error -32602: Invalid request parameters"}
+    events = [_ev(0, "tool_result", tool="Bash", payload_ref="b1")]
+    assert len(_error_snippets(events, blobs)) == 1
+
+
+def test_file_read_snapshot_not_mined():
+    # a Read result of source code containing 'raise ...Error' is not a runtime error
+    blobs = {"b1": "227\t    def f():\n228\t        x = 1\n229\t        raise InfospaceError()\n"}
+    events = [_ev(0, "tool_result", tool="Read", payload_ref="b1")]
+    assert _error_snippets(events, blobs) == []
+
+
 def test_build_digest_includes_error_snippets_and_v2():
     s = Session(session_uid="claude:s", flavor="claude", native_session_id="s", repo="r")
     events = [_ev(0, "user_msg"), _ev(1, "error", payload_ref="b1"), _ev(2, "assistant_msg")]