fix: three provider-related bugs (#8161, #8181, #8147) (#8243)

- Add openai/openai-codex -> openai mapping to PROVIDER_TO_MODELS_DEV
  so context-length lookups use models.dev data instead of 128k fallback.
  Fixes #8161.

- Set api_mode from custom_providers entry when switching via hermes model,
  and clear stale api_mode when the entry has none. Also extract api_mode
  in _named_custom_provider_map(). Fixes #8181.

- Convert OpenAI image_url content blocks to Anthropic image blocks when
  the endpoint is Anthropic-compatible (MiniMax, MiniMax-CN, or any URL
  containing /anthropic). Fixes #8147.

tests/agent/test_models_dev.py

@@ -87,7 +87,10 @@ class TestProviderMapping:
 
     def test_unmapped_provider_not_in_dict(self):
         assert "nous" not in PROVIDER_TO_MODELS_DEV
-        assert "openai-codex" not in PROVIDER_TO_MODELS_DEV
+
+    def test_openai_codex_mapped_to_openai(self):
+        assert PROVIDER_TO_MODELS_DEV["openai"] == "openai"
+        assert PROVIDER_TO_MODELS_DEV["openai-codex"] == "openai"
 
 
 class TestExtractContext: