TST: Add tests for shepard_fallback in test_function_grid.py

[Copilot]

Pull request type

• Code maintenance (refactoring, formatting, tests)

Checklist

• Tests for the changes have been added (if needed)
• Lint (black rocketpy/ tests/) has passed locally
• All tests (pytest tests -m slow --runslow) have passed locally

Current behavior

The test_function_grid.py file tests Function.from_grid() and linear_grid interpolation, but lacks coverage for the shepard_fallback path that triggers when linear_grid is set without a grid interpolator.

New behavior

Added 4 tests for shepard_fallback:

• test_shepard_fallback_warning - Verifies warning emission when fallback triggers
• test_shepard_fallback_2d_interpolation - Validates 2D interpolation matches standard shepard
• test_shepard_fallback_3d_interpolation - Validates 3D interpolation matches standard shepard
• test_shepard_fallback_at_exact_data_points - Confirms exact values at data points

# Example: Testing fallback warning
func = Function(source=[(0,0,0), (1,0,1)], interpolation="shepard")
with warnings.catch_warnings(record=True) as w:
    func.set_interpolation("linear_grid")
    assert "falling back to shepard interpolation" in str(w[0].message)

Breaking change

• No

Additional information

Addresses review feedback from PR #875 requesting test coverage for shepard_fallback.

Original prompt

#875 update test_function_grid.py to include tests for shephard_fallback as well

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.38%. Comparing base (69caf40) to head (beced24).
⚠️ Report is 1 commits behind head on copilot/enhance-drag-curve-functionality.

Additional details and impacted files

@@                             Coverage Diff                              @@
##           copilot/enhance-drag-curve-functionality     #879      +/-   ##
============================================================================
+ Coverage                                     80.25%   80.38%   +0.12%     
============================================================================
  Files                                           104      104              
  Lines                                         12912    12912              
============================================================================
+ Hits                                          10363    10379      +16     
+ Misses                                         2549     2533      -16     

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