Make the `Settings` class hashable

[WonderPG]

Currently some settings sub-classes are not frozen, and some arguments are dictionaries which are not hashable. For instance

class MCPServerConfig(BaseModel):
    """Configuration for a single MCP server."""

    command: str
    args: list[str] | None = None
    env: dict[str, SecretStr] | None = None


class SettingsMCP(BaseModel):
    """Settings for the MCP."""

    servers: dict[str, MCPServerConfig] | None = None

One should find a way to make these classes hashable. For instance @jankrepl 's idea is to do something like this:

class MCPEnv(BaseModel):
    model_config = ConfigDict(frozen=True, extra="allow")

class MCPServerConfig(BaseModel):
    """Configuration for a single MCP server."""

    command: str
    args: tuple[str] | None = None
    env: MCPEnv | None = None

    model_config = ConfigDict(frozen=True)

There might be other ways to do it. To be investigated.