fix(osx): prevent bluetooth connect crashes from missing device metadata.

[giovanninibarbosa]

Summary

This fixes a macOS Bluetooth crash where connect or disconnect notifications can arrive without complete device metadata. The native watcher previously copied NSString.UTF8String values directly into C strings, which could dereference a null pointer before the managed callback was reached.

Tasks

• Harden Bluetooth connect and disconnect watcher callbacks by validating the device address before invoking managed code.
• Add safe native UTF-8 string copying and name fallback handling for optional Bluetooth device names.
• Make paired-device enumeration resilient to missing native name or address strings and clean up partial allocations on failure.
• Add managed-side null pointer handling before converting native UTF-8 strings.

Verification

• git diff --check.
• xcodebuild -project GalaxyBudsClient.Platform.OSX/Native/NativeInterop.xcodeproj -scheme NativeInterop -destination platform=macOS build.

Not Run

• dotnet restore --configfile GalaxyBudsClient/nuget.config GalaxyBudsClient/GalaxyBudsClient.csproj: dotnet is not available in this shell.
• dotnet build GalaxyBudsClient.sln -c Debug: dotnet is not available in this shell.
• dotnet test GalaxyBudsClient.Tests/GalaxyBudsClient.Tests.csproj -c Debug: dotnet is not available in this shell.
• Manual Bluetooth connect/disconnect regression checks.

Notes

There is no dedicated pull request template in docs/ or .github; this draft follows the docs contribution guidance to explain the changes.

[Copilot]

Pull request overview

This PR hardens the macOS Bluetooth native interop path against missing device metadata, especially null native strings in connect/disconnect notifications and paired-device enumeration.

Changes:

• Adds safe Objective-C NSString to UTF-8 C string copying helpers in native Bluetooth code.
• Guards watcher connect/disconnect callbacks against missing addresses and optional names.
• Adds managed-side null pointer handling before converting native UTF-8 strings.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
GalaxyBudsClient.Platform.OSX/Native/src/BluetoothDeviceWatcher.mm	Adds safe string copying and address validation for native connect/disconnect notifications.
GalaxyBudsClient.Platform.OSX/Native/src/Bluetooth.mm	Adds safe enumeration string allocation and cleanup for paired Bluetooth devices.
GalaxyBudsClient.Platform.OSX/BluetoothService.cs	Centralizes native UTF-8 pointer conversion and applies it to enumeration and watcher callbacks.

Comments suppressed due to low confidence (1)

GalaxyBudsClient.Platform.OSX/Native/src/Bluetooth.mm:246

• When addressString is missing, FirstNonEmptyString turns it into an empty string and the enumeration still returns success for that device. That exposes a BluetoothDevice with an empty Address; for example the manual pairing dialog registers the selected device address without revalidating it, and native bt_connect cannot resolve an empty address. Missing addresses should be filtered out or treated as an enumeration failure rather than returned as valid devices.

        NSString *addressString = FirstNonEmptyString([device addressString], nil, nil);
        NSString *nameString = FirstNonEmptyString([device name], [device nameOrAddress], addressString);

        resultDevice->device_name = CopyNSStringToUtf8CString(nameString);
        resultDevice->mac_address = CopyNSStringToUtf8CString(addressString);

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