fix: CapacitorSerial Robustness & Hex Validation

[Erics1337]

Description

This PR improves the robustness of the CapacitorSerial protocol adapter. These changes were extracted from #4725 to be merged independently.

Changes

• Robustness: Added checks for this.pluginAvailable in CapacitorSerial.js methods (loadDevices, connect, disconnect, send, requestPermissionDevice). This prevents crashes when the native BetaflightSerial plugin is missing, such as during browser-based development.
• Validation: Enhanced hexStringToUint8Array to correctly strip optional "0x" prefixes and validate that the input string contains only valid hex characters before parsing.

Testing

• Verified application stability when running in a browser environment without the native serial plugin.
• Verified serial connection still works as expected on supported devices.

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📥 Commits

Reviewing files that changed from the base of the PR and between fece39d and 61f5c65.

📒 Files selected for processing (1)

• src/js/protocols/CapacitorSerial.js (8 hunks)

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• Create PR with unit tests
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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Preview URL: https://pr4729.betaflight-app-preview.pages.dev

[haslinghuis]

• have added some thoughts for improvement in Thoughts on improving serial backend #4722
• my implementation returns early, did you have issues with data format ?

[haslinghuis]

IMO there is no need to add this change, the protocol cancels in the constructor if not available, as the data format has been established.