Supports browser mode for custom provider module

[cenfun]

Clear and concise description of the problem

It doesn't automatically switch to browser mode for custom provider module. for example:

export default defineConfig({
    test: {
        include: ['test/*.test.js'],
        coverage: {
            enabled: true,
            include: ['src/**'],
            
            provider: 'custom',
            customProviderModule: 'vitest-monocart-coverage'

        },

        browser: {
            enabled: true,
            headless: true,
            instances: [{
                browser: 'chromium'
            }],
            provider: 'playwright'
        }
    }
});

It works if the browser mode is manually used

...
customProviderModule: 'vitest-monocart-coverage/browser'
...

Related issue: cenfun/vitest-monocart-coverage#8

Suggested solution

For build-in modules 'v8' and 'istanbul', the solution is below

const provider = options.provider

  if (provider === 'v8' || provider === 'istanbul') {
    let builtInModule = CoverageProviderMap[provider]

    if (provider === 'v8' && loader.isBrowser) {
      builtInModule += '/browser'
    }

    const { default: coverageModule } = await loader.executeId(builtInModule)
...

see source

So can we do something similar for custom module?

  let customProviderModule
  
  // do something here?
  let customModule = options.customProviderModule;
  if (loader.isBrowser) {
      customModule += '/browser'
  }

  try {
    customProviderModule = await loader.executeId(customModule)
  }
  catch (error) {
    throw new Error(
      `Failed to load custom CoverageProviderModule from ${options.customProviderModule}`,
      { cause: error },
    )
  }

Possible questions:
1, What if customProviderModule is a file path like ./path-to/my-file.js?
2, What if customProviderModule does not support browser mode?

Alternative

No response

Additional context

If it cannot be supported, should we consider restructuring the custom module architecture, since vitest@3 has just been released and we still have the opportunity?
I personally think we should separate the code for browser mode and Node.js mode. Otherwise, it will lead to very high complexity and is prone to errors.

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