Enhance MultiTurnSample Validator to Support Multiple Tool Calls (#2017)

- Fixes: #1995 

The current `validate_user_input` method has a strict validation rule
that requires each `ToolMessage` to be immediately preceded by an
`AIMessage` with `tool_calls`. This prevents valid conversation patterns
where:
1. Multiple `ToolMessage` instances appear in sequence
2. `ToolMessage` instances appear after `ToolMessage` types as long as
an `AIMessage` appeared earlier in the conversation

### Changes
This PR modifies the validation logic to:
1. Track whether we've seen an `AIMessage` at any point in the
conversation
2. Allow a `ToolMessage` to follow either an `AIMessage` or another
`ToolMessage`

### Example
The provided sample demonstrates this pattern with:
```python
from ragas.dataset_schema import MultiTurnSample
from ragas.messages import HumanMessage, AIMessage, ToolMessage, ToolCall


sample_input = [
    HumanMessage(
        content="Can you provide me with details about Einstein's theory of relativity?"
    ),
    AIMessage(
        content="Got it! Let me fetch more details from 'General Theory of Relativity by A. Einstein'.",
        tool_calls=[
            ToolCall(
                name="document_retrieve",
                args={"document": "General Theory of Relativity by A. Einstein"},
            ),
            ToolCall(
                name="document_retrieve",
                args={"document": "A. Einstein biography"},
            ),
        ],
    ),
    ToolMessage(
        content="Found relevant documents: 1. Relativity: The Special and the General Theory, 2. General Theory of Relativity by A. Einstein."
    ),
    ToolMessage(content="Found relevant documents: 1. A. Einstein biography"),
    AIMessage(content="I found some documents on Einstein's theory of relativity..."),
]
sample = MultiTurnSample(user_input=sample_input)

```

Author: sahusiddharth

src/ragas/dataset_schema.py

@@ -133,18 +133,34 @@ def validate_user_input(
                 "All inputs must be instances of HumanMessage, AIMessage, or ToolMessage."
             )
 
-        prev_message = None
-        for m in messages:
-            if isinstance(m, ToolMessage):
-                if not isinstance(prev_message, AIMessage):
-                    raise ValueError(
-                        "ToolMessage instances must be preceded by an AIMessage instance."
-                    )
-                if prev_message.tool_calls is None:
+        has_seen_ai_message = False
+
+        for i, m in enumerate(messages):
+            if isinstance(m, AIMessage):
+                has_seen_ai_message = True
+
+            elif isinstance(m, ToolMessage):
+                # Rule 1: ToolMessage must be preceded by an AIMessage somewhere in the conversation
+                if not has_seen_ai_message:
                     raise ValueError(
-                        f"ToolMessage instances must be preceded by an AIMessage instance with tool_calls. Got {prev_message}"
+                        "ToolMessage must be preceded by an AIMessage somewhere in the conversation."
                     )
-            prev_message = m
+
+                # Rule 2: ToolMessage must follow an AIMessage or another ToolMessage
+                if i > 0:
+                    prev_message = messages[i - 1]
+
+                    if isinstance(prev_message, AIMessage):
+                        # Rule 3: If following AIMessage, that message must have tool_calls
+                        if not prev_message.tool_calls:
+                            raise ValueError(
+                                "ToolMessage must follow an AIMessage where tools were called."
+                            )
+                    elif not isinstance(prev_message, ToolMessage):
+                        # Not following AIMessage or ToolMessage
+                        raise ValueError(
+                            "ToolMessage must follow an AIMessage or another ToolMessage."
+                        )
 
         return messages