Return error from NewGTU7 instead of panicking

[Copilot]

The NewGTU7 constructor panics when serial initialization fails, violating Go error handling conventions and making the API unsafe.

Changes

• Changed NewGTU7 signature to return (*GTU7, error)
• Return error on OpenSerial failure instead of panicking
• Updated test to handle error return
• Added test coverage for error case using mock SerialFactory

Example

// Before: panic on failure
gps := NewGTU7(GTU7Config{Serial: cfg})

// After: caller handles error
gps, err := NewGTU7(GTU7Config{Serial: cfg})
if err != nil {
    return fmt.Errorf("failed to initialize GPS: %w", err)
}

Testing

• ✅ TestGTU7_PrefersRMCOverVTG validates success path with Reader injection
• ✅ TestNewGTU7_OpenSerialError validates error propagation when OpenSerial fails
• ✅ All sensor tests pass

Aligns with error handling pattern used in other constructors like NewADS1115.

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[Copilot]

Pull request overview

This pull request refactors the NewGTU7 constructor to follow Go error handling conventions by returning an error instead of panicking when serial initialization fails.

Changes:

• Modified NewGTU7 to return (*GTU7, error) instead of *GTU7
• Replaced panic on OpenSerial failure with proper error propagation
• Updated test to handle the new error return value
• Cleaned up unused dependencies from go.mod and go.sum

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 1 comment.

File	Description
sensors/gtu7.go	Changed function signature to return error and replaced panic with error return
sensors/gtu7_test.go	Updated test to handle error return from NewGTU7
go.mod	Removed unused dependencies and moved golang.org/x/sys to direct dependencies
go.sum	Removed checksums for unused dependencies

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