Migrate from optional-dependencies to dependency-groups

[harupy]

Summary

Migrates the Python SDK from pip's optional-dependencies pattern to uv's dependency-groups pattern for dev dependencies.

Changes

• pyproject.toml: Changed [project.optional-dependencies] to [dependency-groups]
• README.md: Updated installation instructions to use uv sync instead of pip install -e ".[dev]"
• uv.lock: Regenerated with new dependency format

[Copilot]

Pull request overview

This PR migrates the Python SDK’s dev dependency handling from pip/extras (optional-dependencies) to uv’s dependency groups, and updates documentation and lockfile accordingly.

Changes:

• Switches from [project.optional-dependencies] to [dependency-groups] for Python dev dependencies in pyproject.toml.
• Regenerates uv.lock to represent dev dependencies via dev-dependencies and metadata.requires-dev instead of extras.
• Updates the Python README install instructions to reference uv sync rather than the old ".[dev]" extras pattern.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 1 comment.

File	Description
python/pyproject.toml	Defines a dependency-groups.dev group holding the former dev extras, aligning the project with uv’s dependency group model.
python/uv.lock	Reflects the new dev dependency representation ([package.dev-dependencies] and [package.metadata.requires-dev]) for the github-copilot-sdk package.
python/README.md	Updates installation instructions toward uv-based workflows, though the new pip example currently uses an invalid --group flag and needs correction; also note that CONTRIBUTING.md still references the old ".[dev]" extras pattern and should be updated separately.

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[Copilot]

The updated installation snippet mixes pip install with a --group flag, but pip itself doesn’t support --group—that flag is specific to uv and will cause this command to fail. To keep the instructions working after switching to dependency groups, this line should either use a valid pip command (without --group) or be rewritten to use uv in a way that correctly installs the dev group (similar to the uv sync example below).

Suggested change

	pip install -e --group dev .
	pip install -e .

[harupy]

pip >= v25.1 supports --group: https://pip.pypa.io/en/latest/news/#v25-1

[harupy]

@friggeri would you mind taking a look?

[friggeri]

@brettcannon do you mind opining?