Fix dump_config when a model field is None (e.g. fallback_model)

VibeConfig.dump_config used to call tomli_w.dump on the raw dict
returned by model_dump(); tomli_w cannot serialize Python None.
Pydantic's to_jsonable_python(exclude_none=True) only drops Nones
from Pydantic models, not from plain dicts. Add a recursive
_strip_none helper used by dump_config so any Optional[...] field
with a None default round-trips through the TOML file cleanly.

Triggered by the new ModelConfig.fallback_model: str | None field
which broke 21 tests under tests/acp/ that round-trip a config.

Author: obook

albert_code/core/config.py

@@ -341,6 +341,18 @@ def _default_alias_to_name(cls, data: Any) -> Any:
 ]
 
 
+def _strip_none(value: Any) -> Any:
+    """Return a copy of `value` with all None values removed, recursively.
+
+    Used before TOML serialization, since tomli_w refuses None.
+    """
+    if isinstance(value, dict):
+        return {k: _strip_none(v) for k, v in value.items() if v is not None}
+    if isinstance(value, list):
+        return [_strip_none(v) for v in value if v is not None]
+    return value
+
+
 class VibeConfig(BaseSettings):
     active_model: str = "albert-code"
     vim_keybindings: bool = False
@@ -635,8 +647,14 @@ def deep_merge(target: dict, source: dict) -> None:
 
     @classmethod
     def dump_config(cls, config: dict[str, Any]) -> None:
+        # tomli_w cannot serialize Python None values, and Pydantic's
+        # `to_jsonable_python(exclude_none=True)` only drops Nones from
+        # Pydantic models, not from plain dicts that have already been
+        # produced via `model_dump()`. Strip them recursively here so that
+        # any Optional[...] field with a None default round-trips cleanly.
+        cleaned = _strip_none(to_jsonable_python(config, fallback=str))
         with CONFIG_FILE.path.open("wb") as f:
-            tomli_w.dump(config, f)
+            tomli_w.dump(cleaned, f)
 
     @classmethod
     def _migrate(cls) -> None: