Add RS-485 configuration parameters for serial transports

[kevinherron]

Summary

• Add rtsActiveHigh, termination, rxDuringTx, delayBefore, and delayAfter fields to SerialPortTransportConfig for fine-grained RS-485 control (Linux setRs485ModeParameters)
• When rs485Mode is enabled, both client and server transports now call setRs485ModeParameters() with the configured values
• Include builder methods, Javadoc, and a test for default/custom RS-485 configuration

Test plan

• Rs485ConfigTest verifies default values and custom builder configuration
• Manual verification on Linux with an RS-485 adapter

[Copilot]

Pull request overview

Adds full RS-485 parameter support to the serial transport configuration and applies those settings to the underlying jSerialComm SerialPort, with tests validating both config defaults/customization and transport application.

Changes:

• Extend SerialPortTransportConfig with additional RS-485 tuning parameters (RTS polarity, termination, RX-during-TX, delays).
• Apply RS-485 mode parameters to SerialPort in both client and server transports when RS-485 mode is enabled.
• Add unit tests covering defaults, custom values, and transport-to-SerialPort parameter propagation.

Reviewed changes

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

File	Description
modbus-serial/src/main/java/com/digitalpetri/modbus/serial/SerialPortTransportConfig.java	Adds new RS-485 configuration fields + builder setters and documentation.
modbus-serial/src/main/java/com/digitalpetri/modbus/serial/client/SerialPortClientTransport.java	Applies RS-485 parameters to the SerialPort when enabled.
modbus-serial/src/main/java/com/digitalpetri/modbus/serial/server/SerialPortServerTransport.java	Applies RS-485 parameters to the SerialPort when enabled.
modbus-serial/src/test/java/com/digitalpetri/modbus/serial/Rs485ConfigTest.java	Introduces tests for RS-485 config defaults/custom values and transport application via reflection.

Comments suppressed due to low confidence (2)

modbus-serial/src/main/java/com/digitalpetri/modbus/serial/client/SerialPortClientTransport.java:80

• The catch-all exception handler wraps failures from both getCommPort and subsequent configuration calls (e.g., setComPortParameters / setRs485ModeParameters), but the message says "failed to get comm port". This will be misleading if configuration fails. Consider updating the error message (or splitting try/catch) to reflect that port configuration may be the failing step.

            this.serialPort = sp;
          } catch (Exception e) {
            throw new ModbusException(
                "failed to get comm port '%s'".formatted(config.serialPort()), e);
          }

modbus-serial/src/main/java/com/digitalpetri/modbus/serial/server/SerialPortServerTransport.java:88

• The exception message here says "failed to get comm port", but this try block now also includes port configuration (setComPortParameters / setRs485ModeParameters). If configuration throws, the message will point readers at the wrong root cause. Please adjust the message or split error handling so failures are reported accurately.

            this.serialPort = sp;
          } catch (Exception e) {
            throw new ModbusException(
                "failed to get comm port '%s'".formatted(config.serialPort()), e);
          }

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