Add configurable limit on consecutive client response timeouts

[jadamcrain]

• PR Title
Add configurable limit on consecutive client response timeouts

PR Description

Summary

• introduce ClientOptions::max_response_timeouts so applications can cap consecutive Modbus response timeouts
• track consecutive timeouts inside ClientLoop, resetting the counter after any non-timeout response
• surface a new SessionError::MaxTimeouts so TCP/serial tasks drop and reconnect once the limit is reached

Fixes #144.

[gemini-code-assist]

Summary of Changes

Hello @jadamcrain, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the Modbus client's resilience by introducing a configurable mechanism to detect and recover from unresponsive connections. By allowing users to specify a limit on consecutive response timeouts, the system can proactively reset a session, preventing indefinite hangs and improving overall reliability, especially in scenarios where remote devices might silently fail or network issues lead to dropped packets without explicit connection termination signals.

Highlights

• Configurable Timeout Limit: Introduced ClientOptions::max_response_timeouts to allow applications to set a maximum number of consecutive Modbus response timeouts before a session is reset.
• Timeout Tracking: Implemented a TimeoutCounter within the ClientLoop to track consecutive timeouts, which resets upon any successful (non-timeout) response.
• Session Error for Max Timeouts: Added a new SessionError::MaxTimeouts variant, which is triggered when the configured limit is reached, causing TCP/serial tasks to drop the current connection and attempt to reconnect.

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[gemini-code-assist]

Code Review

This pull request introduces a configurable limit on consecutive client response timeouts to enhance connection stability. The implementation for the TCP client is solid and includes good test coverage. However, the feature is currently not functional for serial clients as the configuration is hardcoded to be disabled. Additionally, there's a minor logic issue in the timeout counter that could cause problems in an extreme edge case. The review includes suggestions to address these points.

[gemini-code-assist]

The max_timeouts option is hardcoded to None for the serial client, which disables the new consecutive timeout limit feature. The PR description and the addition of SessionError::MaxTimeouts handling for serial connections suggest this feature should be available for serial clients as well. To fix this, the max_timeouts value should be configurable, likely by passing ClientOptions to SerialChannelTask::new similarly to how it's done for the TCP client.

[gemini-code-assist]

Using wrapping_add here could lead to unexpected behavior in an extreme edge case. If current reaches usize::MAX, it will wrap around to 0, effectively resetting the timeout counter instead of keeping it at its maximum value. This would prevent the MaxTimeouts error from being triggered if the number of timeouts is close to usize::MAX.

Using saturating_add would be safer and more correctly reflect the intent of capping the counter at a maximum value.

Suggested change

	*current = current.wrapping_add(1);
	*current = current.saturating_add(1);