fix(bedrock): make llama2 BOS a string, not a 1-tuple

[Osamaali313]

Problem

messages_to_prompt_llama2 (metagpt/provider/bedrock/utils.py) defines its beginning-of-sequence token as a one-element tuple, not a string:

def messages_to_prompt_llama2(messages: list[dict]) -> str:
    BOS = ("<s>",)          # trailing comma -> tuple, not str
    ...
    prompt = f"{BOS}"       # renders the tuple repr

Because BOS is a tuple, prompt = f"{BOS}" interpolates its repr, so every Llama-2 / Mistral prompt begins with the literal text ('<s>',) instead of the <s> BOS token.

The sibling messages_to_prompt_llama3 in the same file does the same thing correctly with a plain string, which confirms the intent:

def messages_to_prompt_llama3(messages: list[dict]) -> str:
    BOS = "<|begin_of_text|>"   # string
    prompt = f"{BOS}"

The malformed prompt flows through BaseBedrockProvider.get_request_body for the Mistral and Meta (Llama-2) providers in bedrock_provider.py, so every such Amazon Bedrock request was sent a prompt starting with ('<s>',).

Reproduction

BOS = ("<s>",)
prompt = f"{BOS}" + "[INST] hi [/INST]"

	prompt starts with
before (BOS = ("<s>",))	('<s>',)[INST] hi [/INST] — startswith("<s>") → False ❌
after (BOS = "<s>")	<s>[INST] hi [/INST] — startswith("<s>") → True ✅

Fix

Drop the trailing comma so BOS is the string it is used as:

BOS = "<s>"

One character. python -m py_compile passes.

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