fix(providers): keep reasoning items wire-valid for DeepSeek Responses

convert_messages() emitted reasoning items with ``content`` as a plain
string whenever preserve_reasoning was enabled (the DeepSeek spec).
DeepSeek's Responses gateway rejects that shape with a serde error
("input: invalid type: string ..., expected a sequence"), which surfaced
only after token consolidation cleared provider_state and forced the
full-history conversion path; replayed server items already carry list
content, which is why normal multi-turn requests never failed. Serialize
reasoning content as a list of output_text parts, matching the OpenAI
Responses schema and DeepSeek's accepted wire shape (verified live against
api.deepseek.com/responses).

The serde fallback classifier introduced in the previous commit remains as
a last-resort safeguard for any remaining wire incompatibility.

Tests: extend test_preserves_deepseek_reasoning_content to the array shape;
add a full-history regression with the observed failing item, a
replay/consolidation regression covering both replayed and converted
reasoning items, and provider-level request fixtures for both paths.
Full suite: 5773 passed, 22 skipped (only the known local-only
channels/sms packaging failure remains).

nanobot/providers/openai_responses/converters.py

@@ -45,7 +45,7 @@ def convert_messages(
                 if isinstance(reasoning, str) and reasoning:
                     input_items.append({
                         "type": "reasoning",
-                        "content": reasoning,
+                        "content": [{"type": "output_text", "text": reasoning}],
                     })
             if isinstance(content, str) and content:
                 message_id = _unique_item_id(f"msg_{idx}", used_item_ids)