chore: Add thread priority for streaming synchronizer.

[kinyoklion]

Note

Low Risk
Small, localized change that only affects the priority of a background streaming thread; main risk is invalid/out-of-range priorities causing runtime exceptions on some platforms.

Overview
Adds configurable thread priority to the FDv2 streaming synchronizer by passing DataSourceBuildInputs.getThreadPriority() into StreamingSynchronizerImpl and applying it via Thread#setPriority on the stream run thread.

Updates StreamingSynchronizerImpl’s constructor signature accordingly and adjusts all StreamingSynchronizerImplTest instantiations to supply a priority (using Thread.NORM_PRIORITY).

^{Written by Cursor Bugbot for commit 9678f1a. This will update automatically on new commits. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.