fix(postgrest): surface helpful hint for browser DNS failures (ERR_NA…

[Tanusree-samanta]

Summary

Fixes missing error context for ERR_NAME_NOT_RESOLVED and other DNS
failures in browser environments (Chrome, Firefox, Safari).

Relates to: supabase/supabase#46821

Problem

In browsers, DNS failures produce a plain TypeError with no .cause
and no error code — just the message string. supabase-js currently
returns this as an opaque stack trace with no actionable guidance.

• Chrome/Edge: Failed to fetch
• Firefox: NetworkError when attempting to fetch resource.
• Safari: Load failed

Solution

Detect these three browser network-failure messages and populate
error.hint with a developer-friendly message pointing to:

1. The likely root cause (DNS / project URL)
2. The Supabase dashboard (to verify project status)
3. The Supabase status page for outages

Testing

3 new test.each cases added to fetch-errors.test.ts, one per browser.
No behaviour change for Node.js paths or non-DNS errors.

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commit: 539a885