Add flake8 comma rule

[TomeHirata]

I've seen inconsistent comma usage among contributors, so adding flake8-comma rule for consistency. I'm open to not having this rule if we want to keep flexibility.

[Copilot]

Pull Request Overview

This PR integrates the flake8-commas rule to enforce consistent use of trailing commas in multi-line structures and function calls across the codebase.

• Add "COM" to the select list in pyproject.toml to enable flake8-commas.
• Introduce missing trailing commas in multi-line test fixtures and function calls.
• Apply the same formatting update to production code under dspy/, ensuring consistency.

Reviewed Changes

Copilot reviewed 79 out of 79 changed files in this pull request and generated no comments.

File	Description
pyproject.toml	Enable flake8-commas by adding "COM" to the select.
tests/**	Added trailing commas to multi-line tuple/list/dict literals and function arguments in tests.
dspy/**	Added trailing commas to multi-line function calls and data structures in production code.

[chenmoneygithub]

ohh I would not introduce this rule, for example code like this:

        system_user_messages = self.format(  # returns a list of dicts with the keys "role" and "content"
            signature=signature, demos=demos, inputs=inputs,
        )

is against the official python rule PEP8: https://peps.python.org/pep-0008/#when-to-use-trailing-commas, which says it should be broken down into 3 lines if a trailing comma is used.

[TomeHirata]

Thank you Chen, for flagging this. I read the pep8 rule again, and it seems the following example actually does not violate the rule

system_user_messages = self.format( 
    signature=signature, demos=demos, inputs=inputs,
)

The rule states, "it does not make sense to have a trailing comma on the same line as the closing delimiter", therefore this would be a violation while it does not appear in this PR.

system_user_messages = self.format(signature=signature, demos=demos, inputs=inputs,)