fix(models): handle string content when merging consecutive same-role messages

[NIK-TIGER-BILL]

Problem

LiteLLMModel (and any ApiModel subclass) fails with an AssertionError when the input message list contains multiple consecutive messages with the same role (e.g., two system messages in a row):

messages = [
    {"role": "system", "content": "When you say anything Start with 'FOO'"},
    {"role": "system", "content": "When you say anything End with 'BAR'"},
    {"role": "user", "content": "Just say '.'"},
]
response = model(messages)  # AssertionError: Error: wrong content: ...

The prepare_completion_kwargs() helper in models.py merges consecutive same-role messages but asserts that message.content is a list before merging. When messages are provided with plain string content (the common dict format), this assertion fails.

Fix

• Normalize string content to [{"type": "text", "text": ...}] before merging
• Handle the case where the previous output message also has plain string content
• Fix flatten_messages_as_text path to handle string content gracefully

Fixes #1972