test(proxy): cover newline-delimited trimming and sequence access

The bounded chunk buffer had two lines of the newline fallback and the
indexing and equality paths untested. Add tests for both so the patch
is fully covered.

tests/unit/proxy/pass_through_endpoints/test_bounded_chunk_buffer.py

@@ -87,6 +87,35 @@ def test_buffer_memory_stays_bounded_while_a_huge_stream_passes():
     assert buffer.retained_bytes <= 1000
 
 
+def test_buffer_trims_newline_delimited_streams_to_whole_lines():
+    # no blank-line separators: frames are single lines, as in newline-delimited JSON
+    buffer = BoundedChunkBuffer(max_bytes=40)
+    lines = [json.dumps({"n": i}).encode() + b"\n" for i in range(20)]
+
+    for line in lines:
+        buffer.append(line)
+
+    kept = b"".join(buffer)
+    assert buffer.truncated
+    assert kept.startswith(lines[0])
+    assert kept.endswith(lines[-1])
+    assert all(json.loads(part) is not None for part in kept.splitlines())
+
+
+def test_buffer_supports_indexing_slicing_and_equality_with_a_list():
+    buffer = BoundedChunkBuffer(max_bytes=1024)
+    chunks = [b"event: a\ndata: 1\n\n", b"event: b\ndata: 2\n\n"]
+    for chunk in chunks:
+        buffer.append(chunk)
+
+    assert buffer[0] == chunks[0]
+    assert buffer[-1] == chunks[-1]
+    assert buffer[0:2] == chunks
+    assert buffer == chunks
+    assert buffer != chunks[:1]
+    assert buffer != "not a sequence"
+
+
 def _response(chunks: list[bytes]) -> MagicMock:
     response = MagicMock(spec=httpx.Response)
     response.status_code = 200