Add voltage to simple batteries

[rushabhcodes]

Summary

• Add optional voltage to the source_simple_battery schema and SourceSimpleBattery type.
• Add a regression test covering battery voltage parsing.

Root cause

@tscircuit/props already accepts battery.voltage, but the circuit-json battery source schema did not declare the corresponding field. This prevented core from representing the accepted prop in typed source-component JSON.

Validation

• bun test tests/source_simple_battery.test.ts
• bunx tsc --noEmit
• bun run build

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[seveibar]

batteries don't have a consistant voltage- this would be something like nominal_voltage or something- you'll need to do some research to figure out the proper term

[rushabhcodes]

@seveibar how about nominal_voltage