experimental[patch]: Migrate pydantic extra to literals (#25194)

Migrate pydantic extra to literals

Upgrade to using a literal for specifying the extra which is the
recommended approach in pydantic 2.

This works correctly also in pydantic v1.

```python
from pydantic.v1 import BaseModel

class Foo(BaseModel, extra="forbid"):
    x: int

Foo(x=5, y=1)
```

And 


```python
from pydantic.v1 import BaseModel

class Foo(BaseModel):
    x: int

    class Config:
      extra = "forbid"

Foo(x=5, y=1)
```


## Enum -> literal using grit pattern:

```
engine marzano(0.1)
language python
or {
    `extra=Extra.allow` => `extra="allow"`,
    `extra=Extra.forbid` => `extra="forbid"`,
    `extra=Extra.ignore` => `extra="ignore"`
}
```

Resorted attributes in config and removed doc-string in case we will
need to deal with going back and forth between pydantic v1 and v2 during
the 0.3 release. (This will reduce merge conflicts.)


## Sort attributes in Config:

```
engine marzano(0.1)
language python


function sort($values) js {
    return $values.text.split(',').sort().join("\n");
}


class_definition($name, $body) as $C where {
    $name <: `Config`,
    $body <: block($statements),
    $values = [],
    $statements <: some bubble($values) assignment() as $A where {
        $values += $A
    },
    $body => sort($values),
}

```

libs/experimental/langchain_experimental/smart_llm/base.py

@@ -15,7 +15,7 @@ from langchain_core.prompts.chat import (
     HumanMessagePromptTemplate,
 )
 
-from langchain_experimental.pydantic_v1 import Extra, root_validator
+from langchain_experimental.pydantic_v1 import root_validator
 
 
 class SmartLLMChain(Chain):
@@ -84,7 +84,7 @@ class SmartLLMChain(Chain):
     history: SmartLLMChainHistory = SmartLLMChainHistory()
 
     class Config:
-        extra = Extra.forbid
+        extra = "forbid"
 
     # TODO: move away from `root_validator` since it is deprecated in pydantic v2
     #       and causes mypy type-checking failures (hence the `type: ignore`)