Hotfix/fix scene doping limits

[marc-flex]

Greptile Summary

• Fixed doping_bounds() to return [0, 0] instead of sentinel values [1e50, -1e50] when no doping structures are present
• This prevents invalid visualization limits when scenes lack semiconductor doping configurations

Confidence Score: 3/5

• This PR fixes a visualization bug but uses direct float equality which could fail in edge cases
• The fix correctly handles the empty doping case but violates the floating-point comparison best practice (custom rule 4a0cb6d3). Direct equality == 1e50 should use np.isclose() for robustness. The logic is sound but the implementation needs a minor correction.
• Pay attention to tidy3d/components/scene.py - the sentinel value checks need tolerance-based comparison

Important Files Changed

Filename	Overview
tidy3d/components/scene.py	Added sentinel value checks to handle empty doping case, but uses direct float equality instead of tolerance-based comparison

Sequence Diagram

sequenceDiagram
    participant User
    participant Scene
    participant doping_bounds
    participant Structures

    User->>Scene: "Visualize doping properties (N_a, N_d, or doping)"
    Scene->>doping_bounds: "Calculate doping bounds"
    doping_bounds->>Structures: "Loop through all_structures"
    alt Structures with SemiconductorMedium found
        Structures-->>doping_bounds: "Update acceptors_lims and donors_lims"
    else No structures with doping
        Structures-->>doping_bounds: "Limits remain at sentinel values [1e50, -1e50]"
    end
    doping_bounds->>doping_bounds: "Check if limits are still at sentinel values"
    alt Limits are sentinel values
        doping_bounds->>doping_bounds: "Set limits to [0, 0]"
    end
    doping_bounds-->>Scene: "Return acceptors_lims, donors_lims"
    Scene-->>User: "Use limits for visualization"

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Pull Request Overview

This hotfix addresses an issue where the doping_bounds() method would return invalid sentinel values ([1e50, -1e50]) when no doping structures are present in a scene, instead of returning sensible default values of [0, 0].

Key Changes:

• Added sentinel value checks after the doping bounds calculation loop
• Reset bounds to [0, 0] when no doping values are found in the scene

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Diff Coverage

Diff: origin/develop...HEAD, staged and unstaged changes

• tidy3d/components/scene.py (60.0%): Missing lines 2025,2027,2029,2031

Summary

• Total: 10 lines
• Missing: 4 lines
• Coverage: 60%

tidy3d/components/scene.py

  2021                                 if max_value > limits[1]:
  2022                                     limits[1] = max_value
  2023         # make sure we have recorded some values. Otherwise, set to 0
  2024         if np.isinf(acceptors_lims[0]):
! 2025             acceptors_lims[0] = 0
  2026         if np.isinf(acceptors_lims[1]):
! 2027             acceptors_lims[1] = 0
  2028         if np.isinf(donors_lims[0]):
! 2029             donors_lims[0] = 0
  2030         if np.isinf(donors_lims[1]):
! 2031             donors_lims[1] = 0
  2032         return acceptors_lims, donors_lims
  2033 
  2034     def doping_absolute_minimum(self):
  2035         """Get the absolute minimum values of the doping concentrations.

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