Closes #2425.

Problem

request.post_vars() is implemented as dict(parse_qsl(body, keep_blank_values=True)), which silently collapses any duplicate keys to the last value. Forms that submit multiple values for the same field — e.g. <select multiple>, multi-checkbox groups, ?_facet=...&_facet=... style POSTs — lose data with no error. request.args already handles this correctly via MultiParams.

Fix

Return a MultiParams built from parse_qs(...) instead of a dict, matching the shape of request.args. MultiParams was already imported in asgi.py, so no new imports.

Added an items() method to MultiParams that yields (key, first_value) pairs (matching __getitem__ semantics), so MultiParams can be passed directly into dict(), dict-comprehension patterns and similar.

The internal _json_or_form_payload helper feeds two callers — _coerce_execute_write_payload and the QueryStoreView / QueryUpdateView POST handlers — that both had isinstance(data, dict) guards intended to validate JSON shape. Those guards spuriously rejected the new MultiParams on the form path, so they have been gated on is_json. The fall-through logic that re-renders the HTML form on validation errors uses .get() on query_data and works against either dict or MultiParams.

Behavioural notes

• post_vars[key] and post_vars.get(key) continue to work but now return the first submitted value rather than the last. This matches request.args semantics and is documented in the changelog.
• post_vars.getlist(key) now returns the full list of submitted values — the new capability the issue asked for.
• Plugins doing Response.json(await request.post_vars()) should wrap in dict(...) because MultiParams is not directly JSON-serialisable. The bundled my_plugin test fixture has been updated to demonstrate this.

Test plan

• Updated test_request_post_vars to assert MultiParams behaviour against the previously-existing single-value payload.
• New test_request_post_vars_multi mirrors test_request_args, covering [], get, getlist, in, keys, len and KeyError for missing keys.
• Full test suite passes (pytest --ignore=tests/test_multipart.py -n auto: 1853 passed). tests/test_multipart.py has a pre-existing ModuleNotFoundError: multipart_form_data_conformance unrelated to this change.

Files touched

• datasette/utils/asgi.py — post_vars() returns MultiParams.
• datasette/utils/__init__.py — MultiParams.items().
• datasette/views/query_helpers.py — gate isinstance(data, dict) on is_json in _coerce_execute_write_payload.
• datasette/views/stored_queries.py — gate isinstance guards on is_json in QueryStoreView.post and QueryUpdateView.post; drop the now-redundant isinstance(query_data, dict) checks in the error fall-through branches.
• tests/test_internals_request.py — updated and added tests.
• tests/plugins/my_plugin.py — wrap post_vars() in dict() for JSON serialisation.
• docs/internals.rst and docs/changelog.rst — documentation updates.