test: add coverage for WorkContext (context.py)

[mnadzam]

WorkContext in src/fromager/context.py is the central state object passed to virtually every function in the codebase. It has 67% unit test coverage with only 1 explicit test function (test_pip_constraints_args) which tests the pip_constraint_args property. The remaining public methods have no dedicated unit tests.

setup() is exercised indirectly through test fixtures (tmp_context, testdata_context) but has no dedicated test verifying its behavior. package_build_info() has ~39 callers making it the most widely used method.

Scope

Method	Lines	Callers
enable_parallel_builds()	98	1
wheels_build (property)	102	10
pip_wheel_server_args (property)	113	1
uv_clean_cache()	136	1
write_to_graph_to_file()	156	1
package_build_info()	160	43
setup()	169	1
clean_build_dirs()	189	2

Acceptance Criteria

• setup() -- test directory creation and idempotency
• package_build_info() -- test settings delegation with Requirement and string
• enable_parallel_builds() + wheels_build -- test thread-specific path generation
• write_to_graph_to_file() -- test file creation
• pip_wheel_server_args -- test HTTPS and HTTP URL handling
• uv_clean_cache() -- test empty args raises, test correct command and env
• clean_build_dirs() -- test safety check, cleanup enabled, cleanup disabled