fix(core): support (message class, template) tuples in ChatPromptTemplate.from_messages (#33989)

### Description

`ChatPromptTemplate.from_messages` supports multiple tuple formats for
defining message templates. One documented format is `(message class,
template)`, which allows users to specify the message type using the
class directly:

```python
ChatPromptTemplate.from_messages([
    (SystemMessage, "You are a helpful assistant named {name}."),
    (HumanMessage, "{input}"),
])
```

However, this syntax was broken. Passing a tuple like `(HumanMessage,
"{input}")` would raise a Pydantic validation error because the
conversion logic in `_convert_to_message_template` didn't handle
`BaseMessage` subclasses—it only recognized string-based role
identifiers like `"human"` or `"system"`.

This PR adds the missing branch to detect when the first element of a
tuple is a message class (by checking for the `type` class attribute)
and routes it through `_create_template_from_message_type`, which
already knows how to create the appropriate `MessagePromptTemplate` for
each message type.

### Changes

- Updated `_convert_to_message_template` to properly support `(message
class, template)` tuples

### Testing

Added 16 comprehensive unit tests covering:

- Basic usage with `HumanMessage`, `AIMessage`, and `SystemMessage`
classes
- Integration with `invoke()` method
- Mixed syntax (message class tuples alongside string tuples)
- Multiple template variables
- Edge cases: empty templates, static text (no variables)
- Correct extraction of `input_variables`
- Partial variables support
- Combination with `MessagesPlaceholder`
- Mustache template format
- Template operations: `append()`, `extend()`, concatenation, and
slicing
- Special characters and unicode in templates

### Issue

Fixes #33791

### Dependencies

None

---------

Co-authored-by: Mason Daugherty <mason@langchain.dev>

libs/core/langchain_core/prompts/chat.py

@@ -1438,6 +1438,14 @@ def _convert_to_message_template(
             message_ = _create_template_from_message_type(
                 message_type_str, template, template_format=template_format
             )
+        elif (
+            hasattr(message_type_str, "model_fields")
+            and "type" in message_type_str.model_fields
+        ):
+            message_type = message_type_str.model_fields["type"].default
+            message_ = _create_template_from_message_type(
+                message_type, template, template_format=template_format
+            )
         else:
             message_ = message_type_str(
                 prompt=PromptTemplate.from_template(