fix: do not throw when browser mode is used without explicit projects (#9801)

[oilater]

Description

Resolves #9801

This PR fixes an issue where Vitest throws an incorrect validation error when browser mode is enabled without an explicit config.projects, even if valid browser.instances are provided.

Implementation

Updated the browser validation logic to distinguish between workspace and non-workspace configurations.

• When config.projects is not defined, browser mode is allowed as long as valid browser.instances are provided.
• When using a workspace, validation still ensures that at least one project has browser mode enabled.

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[sheremet-va]

This should not be needed because instances inject new projects

[oilater]

Resolved in c6ad238 !

[oilater]

@sheremet-va Could you review this when you have a chance?

[oilater]

I reverted my previous workaround

When no browser section exists in the config file, resolved.browser is falsy in resolveConfig,
so CLI/API browser options are never merged and instances never reach resolveBrowserProjects.

So I fixed this by allowing the merge when resolving the root config (resolved.browser || vitest.vite?.config === viteConfig), restoring browser options only for the root while keeping workspace isolation intact.

Also added a fallback for instance.name since CLI/API-only instances bypass the configResolved hook. Added tests as well.

I'm not sure this is the most fundamental fix, but let me know what you think.