Add Driver helpers for variable lookup

[officialasishkumar]

Adds public Driver helpers for looking up variables by name and retrieving the public HamiltonGraph representation without iterating over list_available_variables().

Changes

• Added a cached Driver.variables mapping keyed by variable name.
• Added Driver.get_variable(name) for direct variable lookup.
• Added Driver.get_graph() to return the external HamiltonGraph API object.
• Added regression coverage for variable lookup and graph access.

How I tested this

• uv run --with pytest pytest tests/test_hamilton_driver.py::test_driver_variable_lookup tests/test_hamilton_driver.py::test_driver_get_graph_returns_hamilton_graph
• uv run ruff format --check hamilton/driver.py tests/test_hamilton_driver.py
• uv run ruff check hamilton/driver.py tests/test_hamilton_driver.py

Notes

Fixes #1138.

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.