fix(tidy3d): SCRF-2777 validate AutoImpedanceSpec in ModeSolver for 2.10.3

[dmarek-flex]

Summary

• Cherry-pick of SCRF-2777 fix from develop onto v2.10.2 for a 2.10.3 patch
• Adds early validation of AutoImpedanceSpec in ModeSolver.validate_pre_upload so that microwave mode solver setup errors (e.g., "No valid isolated conductors found") are caught before upload rather than failing during preprocessing
• Moves the auto impedance validation logic from Simulation into MicrowaveModeSpec._validate_auto_impedance_setup so it can be shared by both Simulation and ModeSolver

Test plan

• CI passes on patch branch
• Verify with SCRF-2777 reproduction case (microstrip line with wide ground plane)

🤖 Generated with Claude Code

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Note

Medium Risk
Changes pre-upload validation behavior for ModeSolver/microwave monitors, which may surface new SetupErrors earlier for previously-uploaded models; logic is localized to validation paths only.

Overview
Adds client-side pre-upload validation for MicrowaveModeSpec when using AutoImpedanceSpec in ModeSolver, so invalid conductor geometries fail locally instead of producing cryptic server-side preprocessing errors.

Refactors the auto-impedance setup check into MicrowaveModeSpec._validate_auto_impedance_setup (reused by both Simulation monitor validation and the new ModeSolver.validate_pre_upload step), and adds a regression test covering the new early failure. Updates the changelog for 2.10.3.

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