feat: add ResponsesApiTransport + wire all Codex transport paths

Add ResponsesApiTransport wrapping codex_responses_adapter.py behind the
ProviderTransport ABC. Auto-registered via _discover_transports().

Wire ALL Codex transport methods to production paths in run_agent.py:
- build_kwargs: main _build_api_kwargs codex branch (50 lines extracted)
- normalize_response: main loop + flush + summary + retry (4 sites)
- convert_tools: memory flush tool override
- convert_messages: called internally via build_kwargs
- validate_response: response validation gate
- preflight_kwargs: request sanitization (2 sites)

Remove 7 dead legacy wrappers from AIAgent (_responses_tools,
_chat_messages_to_responses_input, _normalize_codex_response,
_preflight_codex_api_kwargs, _preflight_codex_input_items,
_extract_responses_message_text, _extract_responses_reasoning_text).
Keep 3 ID manipulation methods still used by _build_assistant_message.

Update 18 test call sites across 3 test files to call adapter functions
directly instead of through deleted AIAgent wrappers.

24 new tests. 343 codex/responses/transport tests pass (0 failures).

PR 4 of the provider transport refactor.

tests/run_agent/test_run_agent.py

@@ -16,6 +16,7 @@ from types import SimpleNamespace
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
+from agent.codex_responses_adapter import _chat_messages_to_responses_input, _normalize_codex_response, _preflight_codex_input_items
 
 import run_agent
 from run_agent import AIAgent
@@ -4248,7 +4249,7 @@ class TestNormalizeCodexDictArguments:
         json.dumps, not str(), so downstream json.loads() succeeds."""
         args_dict = {"query": "weather in NYC", "units": "celsius"}
         response = self._make_codex_response("function_call", args_dict)
-        msg, _ = agent._normalize_codex_response(response)
+        msg, _ = _normalize_codex_response(response)
         tc = msg.tool_calls[0]
         parsed = json.loads(tc.function.arguments)
         assert parsed == args_dict
@@ -4257,7 +4258,7 @@ class TestNormalizeCodexDictArguments:
         """dict arguments from custom_tool_call must also use json.dumps."""
         args_dict = {"path": "/tmp/test.txt", "content": "hello"}
         response = self._make_codex_response("custom_tool_call", args_dict)
-        msg, _ = agent._normalize_codex_response(response)
+        msg, _ = _normalize_codex_response(response)
         tc = msg.tool_calls[0]
         parsed = json.loads(tc.function.arguments)
         assert parsed == args_dict
@@ -4266,7 +4267,7 @@ class TestNormalizeCodexDictArguments:
         """String arguments must pass through without modification."""
         args_str = '{"query": "test"}'
         response = self._make_codex_response("function_call", args_str)
-        msg, _ = agent._normalize_codex_response(response)
+        msg, _ = _normalize_codex_response(response)
         tc = msg.tool_calls[0]
         assert tc.function.arguments == args_str