Fix parseLatLon to detect and return strconv.ParseFloat errors

[Copilot]

The parseLatLon function silently ignored strconv.ParseFloat errors, causing invalid coordinate strings to return (0, 0) — valid coordinates in the Gulf of Guinea — masking parse failures.

Changes:

• Check and return errors from both latitude and longitude parsing with wrapped error messages
• Add fmt import for error formatting
• Add 7 test cases covering empty strings, invalid inputs, and hemisphere variations

Before:

la, _ := strconv.ParseFloat(lat, 64)
lo, _ := strconv.ParseFloat(lon, 64)

After:

la, err := strconv.ParseFloat(lat, 64)
if err != nil {
    return 0, 0, fmt.Errorf("parse latitude: %w", err)
}
lo, err := strconv.ParseFloat(lon, 64)
if err != nil {
    return 0, 0, fmt.Errorf("parse longitude: %w", err)
}

Callers in parseGPGGA and parseGPRMC already handle the error return appropriately.

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[Copilot]

Pull request overview

This PR fixes a bug in the parseLatLon function where strconv.ParseFloat errors were silently ignored, causing invalid coordinate strings to return (0, 0) instead of propagating the error. This masked parse failures since (0, 0) represents valid coordinates in the Gulf of Guinea.

Changes:

• Added proper error handling for latitude and longitude parsing with descriptive wrapped error messages
• Improved the empty string error message for clarity
• Added the fmt import for error formatting
• Added a new TestParseLatLon test function with 7 test cases

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
sensors/gtu7.go	Fixed error handling in parseLatLon to check and return strconv.ParseFloat errors with wrapped messages; improved empty string error message
sensors/gtu7_test.go	Added TestParseLatLon test function with test cases for valid coordinates, empty strings, and invalid inputs (incomplete implementation)

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