Fix serial port resource leak in GTU7 sensor

[Copilot]

The GTU7 Run method opens a serial port but never closes it, leaking file descriptors.

Changes

• Changed GTU7.r field type: io.Reader → io.ReadCloser to retain Close() access while reflecting actual usage (GTU7 only reads, never writes)
• Added cleanup defer: Close serial port on Run exit, matching VH400's ADC cleanup pattern
• Test compatibility: Wrap test readers with stdlib io.NopCloser instead of custom implementation

func (g *GTU7) Run(ctx context.Context) error {
    defer close(g.out)
    defer func() {
        _ = g.r.Close()
    }()
    // ... rest of implementation
}

The io.NopCloser wrapper from the standard library allows strings.NewReader and similar test fixtures to work unchanged while production code using SerialPort gets proper cleanup. Using io.ReadCloser instead of io.ReadWriteCloser makes the intent clearer and reduces unnecessary custom code.

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[Copilot]

Pull request overview

This pull request fixes a resource leak in the GTU7 GPS sensor where the serial port was opened but never closed. The fix adds proper cleanup by deferring the close operation and changes the reader field type to retain access to the Close method.

Changes:

• Changed GTU7's reader field from io.Reader to io.ReadWriteCloser to enable cleanup
• Added deferred cleanup in Run() method to close the serial port on exit
• Introduced nopCloser wrapper to maintain test compatibility with readers that don't implement Close

Comments suppressed due to low confidence (1)

sensors/gtu7.go:73

• The code that checks and defaults the cfg.Buf field (previously at lines 55-58) has been accidentally removed. This breaks the buffer size defaulting behavior that is tested in gtu7_test.go. When cfg.Buf is 0 or negative, the tests expect it to default to 16, but now it will create a channel with capacity 0, causing the tests to fail.

	var r io.ReadWriteCloser
	if cfg.Reader != nil {
		// Wrap test reader in a nopCloser if it doesn't implement ReadWriteCloser
		if rc, ok := cfg.Reader.(io.ReadWriteCloser); ok {
			r = rc
		} else {
			r = nopCloser{cfg.Reader}
		}
	} else {
		port, err := cfg.Factory.OpenSerial(cfg.Serial)
		if err != nil {
			panic(err)
		}
		r = port
	}

	return &GTU7{
		name: cfg.Name,
		out:  make(chan GPSFix, cfg.Buf),

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