[Repo Assist] test(utils): add tests for regression and graph_operations utility modules

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Summary

Adds test coverage for two previously untested utility modules in dowhy/utils/:

File	Functions covered
tests/utils/test_regression.py	get_numeric_features, generate_moment_function, create_polynomial_function
tests/utils/test_graph_operations.py	adjacency_matrix_to_adjacency_list, find_ancestor, induced_graph, get_simple_ordered_tree, is_connected, convert_to_undirected_graph, add_edge, del_edge, str_to_dot, daggity_to_dot

Key highlights

Bug caught by new tests: closure bug in create_polynomial_function

The test test_degree_zero_is_constant_one (and test_each_function_computes_correct_power) expose a pre-existing closure bug where every function in the list returned by create_polynomial_function(n) computed x^n instead of x^0, x^1, ..., x^n. This PR includes the fix (a one-line default-argument binding).

# Without fix: all functions return x^max_degree
fns = create_polynomial_function(4)
fns[0](x)  # returns x^4, not x^0 = 1 !

This bug silently corrupted the polynomial basis used by NonParametricSensitivityAnalyzer in causal_refuters/reisz.py.

graph_operations edge-case coverage

• add_edge correctly skips edges that would introduce a cycle.
• del_edge correctly restores edges that would disconnect the graph.
• Ancestor traversal and induced-subgraph extraction are verified on small, known graphs.

Rationale

These utilities underpin core causal inference functionality (sensitivity analysis, graph operations) but had zero direct test coverage. The new tests are simple, fast, and require no heavy optional dependencies.

Test Status

✅ flake8 . --count --select=E9,F63,F7,F82 — no hard errors
✅ black --check — no formatting changes needed
✅ isort --check — import order unchanged
⚠️ Full test suite not runnable in this environment (no installed Python package env); test correctness was validated by manual execution of the logic.

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Pull request overview

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Adds unit tests for previously untested dowhy/utils modules and includes a small bug fix uncovered by the new regression tests.

Changes:

• Add new test suites for regression and graph_operations utility functions.
• Fix a closure-capture bug in create_polynomial_function so each generated term uses the correct degree.
• Validate key graph utility edge cases (cycle prevention, connectivity preservation, DAGGity conversion).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
tests/utils/test_regression.py	Adds tests for numeric feature detection, moment function, and polynomial basis generation (incl. regression case for closure bug).
tests/utils/test_graph_operations.py	Adds tests for adjacency conversion, ancestor traversal, induced subgraphs, connectivity, edge edits, and DOT conversion helpers.
dowhy/utils/regression.py	Fixes polynomial term generation by binding loop variable via default argument.

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