test: add misbehaving mock ClickHouse harness (#194)

test/mock/README.md

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+# Misbehaving Mock ClickHouse
+
+Tests that exercise pg_clickhouse against intentionally malformed ClickHouse
+HTTP responses. Companion to issue [#194].
+
+## What it covers
+
+The real ClickHouse server is well-behaved, so the parser (`src/parser.c`)
+and HTTP transport (`src/http.c`, `src/http_streaming.c`) rarely see invalid
+input in CI. PR [#188] showed there are real bugs lurking when the bytes on
+the wire are not the bytes we expect. This mock returns those bytes on
+demand so we can lock the behavior down.
+
+Behaviors covered:
+
+- `http_500` — ClickHouse-style server error with `X-ClickHouse-Exception-Code`.
+- `http_503` — Service unavailable with no body.
+- `invalid_array` — TSV row whose final field opens an array literal but
+  never closes it (the bug class fixed in #188).
+- `truncated_chunked` — chunked transfer that promises 32 bytes and ships 5,
+  then closes without the terminating chunk.
+- `slow_headers` — delays the first byte to exercise client-side timeouts.
+- `invalid_json` — `application/json` content type with a truncated JSON body.
+- `drop_during_body` — sends a `Content-Length` then drops the socket
+  partway through the body.
+- `trailing_garbage` — valid TSV followed by 32 bytes of binary smear and no
+  trailing newline.
+- `unterminated_string` — array field with an open quote and no closing quote.
+- `backslash_at_eof` — field that ends with a lone backslash (escape with no
+  escapee).
+- `wrong_content_length` — `Content-Length` larger than the body actually
+  delivered.
+
+## Running the tests
+
+The mock and its tests are pure-Python and have no PostgreSQL or ClickHouse
+dependency.
+
+```sh
+# from the repo root
+pip install pytest
+python3 -m pytest test/mock -v
+```
+
+To run the mock standalone (e.g. when wiring it into a future regression
+test):
+
+```sh
+python3 test/mock/mock_clickhouse.py --port 18123
+```
+
+Then point pg_clickhouse at it:
+
+```sql
+SELECT clickhouse_raw_query(
+    '-- BEHAVIOR=http_500
+SELECT 1',
+    'host=127.0.0.1 port=18123'
+);
+```
+
+The pg-side regression test that drives the mock through `clickhouse_raw_query()`
+lives at `test/mock/mock_misbehaving.sql`. It is *not* part of the default
+`make installcheck` schedule (the mock server has to be running first); see
+the comment at the top of that file for how to opt in.
+
+[#188]: https://github.com/ClickHouse/pg_clickhouse/pull/188
+[#194]: https://github.com/ClickHouse/pg_clickhouse/issues/194

test/mock/conftest.py

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+"""Pytest configuration: add this directory to ``sys.path`` so the test file
+can ``import mock_clickhouse`` without an installed package."""
+
+from __future__ import annotations
+
+import os
+import sys
+
+HERE = os.path.dirname(os.path.abspath(__file__))
+if HERE not in sys.path:
+    sys.path.insert(0, HERE)