I tried this config, it seem throw Error:

Error: Plugin context is not set
    at PluginContainer.getContext

Then I check source code, it real exist a logic error:

1. buildStarted will check context
   

2. setContext method happend in runEsbuild:
   

3. buildStarted invoke before runEsbuild, while exec buildStarted, it not yet set context, so it should throw error: "Plugin context is not set"
   

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