Support custom sdkRoot for frontend server executable paths

[nielsenko]

The sdkRoot parameter was already accepted but wasn't actually used when resolving executable and snapshot paths. All paths were unconditionally derived from Platform.resolvedExecutable via sdkDir, ignoring any custom sdkRoot passed by the caller.

This fix wires sdkRoot through to the path helpers so that when a custom SDK root is provided, all resolved paths (dart executable, AOT runtime, AOT snapshot, and AppJIT snapshot) use it instead of the host process's SDK location.

Changes:

• Converted _dartAotRuntimePath, _feServerAotSnapshotPath, and _feServerAppJitSnapshotPath from static final fields to functions that accept an optional sdkRoot, falling back to sdkDir when not provided.
• Added _dartExecutable(sdkRoot) helper that returns the custom SDK's bin/dart when sdkRoot is set, otherwise Platform.resolvedExecutable.
• Updated all Process.start calls to use the resolved paths.

Fixes: #2767

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