feat: allow custom GridSpec in tidy3d write_sparameters

[vvahidd]

Adds an optional grid_spec: td.GridSpec | None parameter to Tidy3DComponent.get_component_modeler, write_sparameters, and write_sparameters_batch, giving full control over the simulation mesh (e.g. td.GridSpec.uniform(dl=20*nm) or per-axis AutoGrid).

When grid_spec is None (default), behavior is unchanged: it falls back to td.GridSpec.auto(wavelength=wavelength, min_steps_per_wvl=min_steps_per_wvl). min_steps_per_wvl is ignored when grid_spec is provided.

Summary by Sourcery

Add support for user-provided Tidy3D GridSpec when generating component modelers and writing s-parameters, while preserving existing auto grid behavior by default.

New Features:

• Allow passing a custom td.GridSpec to get_component_modeler for explicit control over the simulation mesh.
• Expose the grid_spec parameter in write_sparameters and write_sparameters_batch so s-parameter simulations can use user-defined meshing.

Enhancements:

• Clarify that min_steps_per_wvl is ignored when a custom grid_spec is provided and document the default auto GridSpec behavior in the public API.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

This PR adds an optional grid_spec parameter to Tidy3D-related APIs so callers can fully control the simulation mesh, while preserving the existing auto-grid behavior when grid_spec is not provided.

Sequence diagram for grid_spec handling in write_sparameters

sequenceDiagram
    participant Caller
    participant write_sparameters
    participant get_component_modeler
    participant td_GridSpec as td.GridSpec

    Caller->>write_sparameters: write_sparameters(..., grid_spec)
    write_sparameters->>get_component_modeler: get_component_modeler(..., grid_spec)
    alt grid_spec is not None
        get_component_modeler->>get_component_modeler: use provided grid_spec
    else grid_spec is None
        get_component_modeler->>td_GridSpec: auto(wavelength, min_steps_per_wvl)
        td_GridSpec-->>get_component_modeler: GridSpec
    end

Loading

File-Level Changes

Change	Details	Files
Add optional grid_spec parameter to component modeling and s-parameter writing APIs, using it when provided and falling back to the existing auto grid behavior otherwise.	Extend get_component_modeler signature with a grid_spec parameter defaulting to None and pass it through to the grid construction logic. Change grid construction in get_component_modeler to use the provided grid_spec when not None, otherwise call td.GridSpec.auto with wavelength and min_steps_per_wvl. Update docstrings for get_component_modeler, write_sparameters, and write_sparameters_batch to document the new grid_spec parameter and clarify that min_steps_per_wvl is ignored when grid_spec is provided. Extend write_sparameters and write_sparameters_batch signatures with the grid_spec parameter and forward it into get_component_modeler. Clarify default mesh behavior in the public API documentation to ensure backward compatibility when grid_spec is omitted.	gplugins/tidy3d/component.py

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[github-actions]

Label error. Requires at least 1 of: breaking, bug, github_actions, documentation, dependencies, enhancement, feature, maintenance, security. Found:

[sourcery-ai]

Hey - I've found 2 issues, and left some high level feedback:

• In write_sparameters_batch, grid_spec is documented but not added to the function signature or passed through to get_component_modeler, which will confuse users expecting it to work; consider wiring the parameter through consistently or removing it from the docstring.
• Silently ignoring min_steps_per_wvl when grid_spec is provided may be surprising; consider adding a small runtime check or warning when both are passed to make this behavior explicit to callers.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- In `write_sparameters_batch`, `grid_spec` is documented but not added to the function signature or passed through to `get_component_modeler`, which will confuse users expecting it to work; consider wiring the parameter through consistently or removing it from the docstring.
- Silently ignoring `min_steps_per_wvl` when `grid_spec` is provided may be surprising; consider adding a small runtime check or warning when both are passed to make this behavior explicit to callers.

## Individual Comments

### Comment 1
<location path="gplugins/tidy3d/component.py" line_range="245" />
<code_context>
             wavelength: The wavelength for the ModalComponentModeler. Defaults to 1.55.
             bandwidth: The bandwidth for the ModalComponentModeler. Defaults to 0.2.
             num_freqs: The number of frequencies for the ModalComponentModeler. Defaults to 21.
-            min_steps_per_wvl: The minimum number of steps per wavelength for the ModalComponentModeler. Defaults to 30.
+            min_steps_per_wvl: The minimum number of steps per wavelength for the ModalComponentModeler. Defaults to 30. Ignored when grid_spec is provided.
</code_context>
<issue_to_address>
**issue:** Docstring default for `num_freqs` is inconsistent with the function signature.

In `get_component_modeler`, the parameter `num_freqs` defaults to `6`, but the docstring still says `Defaults to 21.` Please update the docstring so it matches the actual default value.
</issue_to_address>

### Comment 2
<location path="gplugins/tidy3d/component.py" line_range="281-283" />
<code_context>
         )

-        grid_spec = td.GridSpec.auto(
+        grid_spec = grid_spec or td.GridSpec.auto(
             wavelength=wavelength, min_steps_per_wvl=min_steps_per_wvl
         )

</code_context>
<issue_to_address>
**suggestion:** Use an explicit `is None` check for `grid_spec` instead of `grid_spec or ...`.

Using `grid_spec or td.GridSpec.auto(...)` depends on `grid_spec` always being truthy. If `GridSpec` (or a wrapper) ever implements a falsy `__bool__`/`__len__`, the provided spec would be incorrectly replaced. An explicit `if grid_spec is None` check avoids this potential misbehaviour and is safer.

```suggestion
        if grid_spec is None:
            grid_spec = td.GridSpec.auto(
                wavelength=wavelength, min_steps_per_wvl=min_steps_per_wvl
            )
```
</issue_to_address>

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[gemini-code-assist]

Code Review

This pull request introduces a grid_spec parameter to get_component_modeler and write_sparameters in gplugins/tidy3d/component.py, allowing users to provide a custom grid specification for the simulation mesh. Feedback on this change highlights that the custom grid_spec is not forwarded to the get_component_modeler call used for plotting the simulation, meaning the plotted simulation will not accurately reflect the custom grid configuration.

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