feat(supabase): add failOnNetworkError option to prevent silent fallback to anon key

[mandarini]

Summary

• Adds failOnNetworkError option to prevent silent fallback to anon key during network failures
• When enabled, throws instead of silently degrading auth, giving apps clear error handling
• Backward compatible: defaults to false (current behavior)

Problem

During network issues (Cloudflare outages, DNS failures, timeouts), getSession() returns { session: null, error }. The client ignores the error and uses the anon key, causing:

• RLS policies with auth.uid() = user_id to fail (auth.uid() is NULL)
• Users receive cryptic 406 errors instead of network errors

Solution

const supabase = createClient(url, key, {
  auth: { failOnNetworkError: true }
})

[coveralls]

coverage: 80.691% (-0.5%) from 81.176%
when pulling cded2de on feat/fail-on-network-error
into af85057 on master.

[mandarini]

Closing after discussion with @hf