fix(immutable): honor hook-level refreshInterval in useSWRImmutable (#4322)

[vaibhavmashal]

Summary

In #4208, \immutable\ middleware was updated to set \config.refreshInterval = 0\ to prevent inheriting global \SWRConfig\
efreshInterval. However, because this was unconditionally executed inside the middleware, it ignored explicit hook-level
efreshInterval\ options passed directly to \useSWRImmutable\ (e.g. \useSWRImmutable(key, fetcher, { refreshInterval: 100 })).

Changes

• Removed unconditional \config.refreshInterval = 0\ from the \immutable\ middleware in \src/immutable/index.ts.
• Defined \useSWRImmutable\ with default
  efreshInterval: 0\ in \hookConfig\ ({ refreshInterval: 0, ...config, use: (config.use || []).concat(immutable) }) so that global config polling is ignored while explicit hook options are preserved.
• Added a unit test in \ est/use-swr-immutable.test.tsx\ verifying hook-level
  efreshInterval\ revalidations.

Closes #4322

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