Add optional extras to slim the core dependency set

[levon003]

What

Trims FlexEval's core dependency set so a plain pip install python-flexeval is substantially leaner. Three optional, user-facing features now live behind extras, and several dependencies that were only needed for tests/dev have moved to the dev group (or been dropped).

Partially addresses #87.

Optional extras

Extra	Package	Enables
litellm	litellm	litellm_completion — routing completions to many non-OpenAI providers
langgraph	langgraph-checkpoint	the langgraph_sqlite data source (loading langgraph checkpoint DBs)
viz	matplotlib	the visualize_graph helper (metric dependency-graph plots)

Install with e.g. pip install 'python-flexeval[litellm]' (or combine: [litellm,langgraph,viz]).

Dependency moves

• Dropped entirely: langchain — nothing in the repo imports it.
• Moved to the dev group (test/fixture-only): langchain-openai, langgraph, langgraph-checkpoint-sqlite (unit tests + fixtures), and sympy (integration-test fixtures only).
• Each extra's package is also kept in dev (or pulled in transitively) so a plain uv sync still gives contributors the full feature set and CI can exercise everything.

How it works

• Lazy imports. Each optional dependency is imported inside the function that needs it, with a friendly error pointing at the right extra:
  • litellm → inside litellm_completion (completion_functions.py). It was previously a top-level import in a module imported eagerly by core code, so without this the base install would break for everyone.
  • langgraph-checkpoint (JsonPlusSerializer) → inside load_langgraph_sqlite (data_loader.py). Everything else referencing "langgraph" in src/ is peewee field names / enum values, not package imports — so this single wrapped import is sufficient.
  • matplotlib → inside visualize_graph (helpers.py), already defensive; error message now points at the viz extra.
• visualize_graph tidied while in there: accepts an optional ax, returns (fig, ax) so it composes into a caller's figure, and the inline node-label construction moved into _graph_node_label.
• CI: validate.yaml now runs uv sync --all-extras so every extra is tested (and stays future-proof as more deps become optional).
• Docs: getting_started.rst documents each extra's install command.

Verification

• uv run python -m unittest discover -s tests.unit → 67 pass (6 pre-existing expected failures); ruff clean. Green in CI via uv sync --all-extras.
• Simulated each dependency being absent: flexeval imports fine, and the relevant function raises a clear ImportError with the correct pip install 'python-flexeval[...]' hint.
• visualize_graph smoke-tested: returns (fig, ax), honors a passed-in ax, and still writes output_path.
• Built-wheel metadata confirms litellm / langgraph / viz are opt-in extras and that langchain / langgraph / langgraph-checkpoint-sqlite / sympy are no longer core Requires-Dist.

Upgrade note

These are user-visible install changes: anyone currently relying on litellm, langgraph checkpoint loading, or visualize_graph from a plain pip install python-flexeval will need to add the relevant extra after this ships. Worth a line in the release notes for the next version bump.

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