chore: improve spyOn ESM namespace error guidance

[brightcat1]

Description

Resolves #9467

Improve vi.spyOn error guidance when spying on native ESM module namespace exports (non-configurable).
In Browser Mode, the message now points to the Browser "Spying on Module Exports" limitations section.
In Node/SSR, it instead suggests inlining dependencies via "test.server.deps.inline" and using module mocking docs.
Also updated docs/guide/migration.md with a short v4 migration note for this case.

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Tests

• Run the tests with pnpm test:ci.

Documentation

• If you introduce new functionality, document it. You can run documentation with pnpm run docs command.
  • Updated docs/guide/migration.md (v4 migration note for vi.spyOn on native ESM namespace exports).

Changesets

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