fix(core): preserve stop sequences from model_config entity in easy UI apps (#41460)

[loulanyue]

Summary

Fixes #41460.

In non-workflow (easy UI based) apps such as Chatbot, Agent, and Completion apps, stop sequences configured in model parameters were never being passed through to the model execution runtime.

Root Cause

1. ModelConfigManager.convert extracts stop from completion_params and assigns it to ModelConfigEntity.stop:

   completion_params = model_config.get("completion_params") or {}
   stop = []
   if "stop" in completion_params:
       stop = completion_params["stop"]
       del completion_params["stop"]
   
   return ModelConfigEntity(..., parameters=completion_params, stop=stop)

2. However, downstream ModelConfigConverter.convert previously only inspected model_config.parameters (which no longer contained "stop"), completely ignoring model_config.stop:

   # Previous:
   completion_params = model_config.parameters
   stop = []
   if "stop" in completion_params:
       stop = completion_params["stop"]
       del completion_params["stop"]

   As a consequence, stop was always set to an empty list [] in ModelConfigWithCredentialsEntity, causing the model to generate past configured stop sequences.

Solution

• Updated ModelConfigConverter.convert to initialize stop from model_config.stop (list(model_config.stop) if getattr(model_config, "stop", None) else []), while still supporting fallback extraction from completion_params["stop"] if present.
• Avoided in-place mutation of model_config.parameters by taking a dictionary copy completion_params = dict(model_config.parameters).
• Added unit tests in test_model_config_converter.py covering model_config.stop and end-to-end ModelConfigManager -> ModelConfigConverter conversion.

Verification

• uv run pytest tests/unit_tests/core/app/app_config/easy_ui_based_app/test_model_config_converter.py (15 passed)
• uv run pytest tests/unit_tests/core/app/app_config/easy_ui_based_app/test_model_config_manager.py (21 passed)
• uv run ruff check (0 errors)
• uv run ruff format --check (2 files already formatted)

[github-actions]

Pyrefly Diff

base → PR

--- /tmp/pyrefly_base.txt	2026-08-29 14:00:36.662230200 +0000
+++ /tmp/pyrefly_pr.txt	2026-08-29 14:00:28.362198639 +0000
@@ -4192,6 +4192,8 @@
    --> tests/unit_tests/core/app/app_config/easy_ui_based_app/test_dataset_manager.py:205:47
 ERROR Object of class `NoneType` has no attribute `dataset_ids` [missing-attribute]
    --> tests/unit_tests/core/app/app_config/easy_ui_based_app/test_dataset_manager.py:206:16
+ERROR Argument `dict[str, dict[str, str | dict[str, float | list[str]]]]` is not assignable to parameter `config` with type `AppModelConfigDict` in function `core.app.app_config.easy_ui_based_app.model_config.manager.ModelConfigManager.convert` [bad-argument-type]
+   --> tests/unit_tests/core/app/app_config/easy_ui_based_app/test_model_config_converter.py:123:45
 ERROR Missing required key `opening_statement` for TypedDict `AppModelConfigDict` [bad-typed-dict-key]
   --> tests/unit_tests/core/app/app_config/easy_ui_based_app/test_model_config_manager.py:72:40
 ERROR Missing required key `suggested_questions` for TypedDict `AppModelConfigDict` [bad-typed-dict-key]

[github-actions]

Pyrefly Type Coverage

Metric	Base	PR	Delta
Type coverage	61.46%	61.46%	-0.01%
Strict coverage	61.06%	61.06%	-0.01%
Typed symbols	42,528	42,527	-1
Untyped symbols	26,840	26,846	+6
Modules	3277	3277	0