fix(test): use public API in async regression test; fix PS3 metadata order (ITL-616)

Switch test_async_three_fetches_no_content_hash_leak to call src.keys()
and src.output_schema() instead of accessing the private _accumulated_stream
attribute. Also reorder PS3 metadata block in DESIGN_ISSUES.md to match
the file-wide convention (Status, Severity, Issue then Fix at end of body).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: kurodo3[bot]

DESIGN_ISSUES.md

@@ -126,7 +126,6 @@ lock before iterating, so later writes to the stream don't affect the snapshot m
 
 ### PS3 — `_combine` leaks `_content_hash` into the data schema on the second accumulating fetch
 **Status:** resolved
-**Fix:** Added `_STREAM_COMBINE_COLUMNS = ColumnConfig(system_tags=True, source=True, context=True)` constant and replaced `as_table(all_info=True)` with `as_table(columns=_STREAM_COMBINE_COLUMNS)` in `_combine`. `content_hash` is intentionally excluded — it is a synthetic output column, never a stored one.
 **Severity:** high
 **Issue:** ITL-616
 
@@ -147,9 +146,7 @@ On the next `_combine` call, `_validate_combining_schemas` compares:
 This raises `SchemaInconsistencyError`. A polling source emitting one new row per poll will
 change its data schema on the second new-data poll and crash on the third.
 
-**Fix:** Replace `all_info=True` with a named module-level constant
-`_STREAM_COMBINE_COLUMNS = ColumnConfig(system_tags=True, source=True, context=True)`.
-`content_hash` is intentionally absent — it is a synthetic output column, never a stored one.
+**Fix:** Added `_STREAM_COMBINE_COLUMNS = ColumnConfig(system_tags=True, source=True, context=True)` constant and replaced `as_table(all_info=True)` with `as_table(columns=_STREAM_COMBINE_COLUMNS)` in `_combine`. `content_hash` is intentionally excluded — it is a synthetic output column, never a stored one.
 
 ---
 

tests/test_channels/test_polling_source.py

@@ -1129,12 +1129,11 @@ async def test_async_three_fetches_no_content_hash_leak(self):
             items.append((tag, data))
 
         assert len(items) == 3
-        assert src._accumulated_stream is not None
 
-        _, data_keys = src._accumulated_stream.keys()
+        _, data_keys = src.keys()
         assert "_content_hash" not in data_keys
 
-        _, data_schema = src._accumulated_stream.output_schema()
+        _, data_schema = src.output_schema()
         assert "_content_hash" not in data_schema