fix: add reasoning_echo_flag to init snapshot and switch rollback

Address review feedback on PR #76503:

1. Init-time primary snapshot (agent_init.py:2756) was missing
   reasoning_echo_flag — after fallback recovery the flag was
   restored as False even when model.reasoning_echo: true was set.

2. Switch transaction snapshot (agent_runtime_helpers.py:2284) was
   missing _reasoning_echo_flag — a failed client rebuild during
   switch_model would leave the old provider with the new provider
   echo policy.

Both omissions now fixed. No test regressions (56 passed).

Signed-off-by: Yingliang Zhang <zhangyingliang@outlook.com>

agent/agent_init.py

@@ -2961,6 +2961,7 @@ def init_agent(
         "client_kwargs": dict(agent._client_kwargs),
         "use_prompt_caching": agent._use_prompt_caching,
         "use_native_cache_layout": agent._use_native_cache_layout,
+        "reasoning_echo_flag": getattr(agent, "_reasoning_echo_flag", False),
         # Context engine state that _try_activate_fallback() overwrites.
         # Use getattr for model/base_url/api_key/provider since plugin
         # engines may not have these (they're ContextCompressor-specific).

agent/agent_runtime_helpers.py

@@ -2654,6 +2654,7 @@ def switch_model(agent, new_model, new_provider, api_key='', base_url='', api_mo
             "_anthropic_base_url",
             "_is_anthropic_oauth",
             "_config_context_length",
+            "_reasoning_echo_flag",
         )
     }
     # _client_kwargs is a dict — snapshot a shallow copy so mutating the