refactor(gateway): switch defaults to autoconf.FallbackDNSResolvers

[lidel]

This PR moves hardcoded DNS resolver defaults from gateway.NewDNSResolver to autoconf.FallbackDNSResolvers so we have them in one place, while preserving existing behavior for users.

Changes

• Move hardcoded DNS resolver defaults to autoconf.FallbackDNSResolvers
• NewDNSResolver(nil) uses autoconf.FallbackDNSResolvers (preserves .eth resolution)
• NewDNSResolver(map[string]string{}) explicitly uses system DNS only
• Remove autoconf.Client.ExpandDNSResolvers() method (no use case yet)
• Update test to use .foo instead of .eth TLD
• No network calls at gateway initialization (privacy-preserving)

Migration

No migration needed. For those wanting custom DNS configuration, existing code (Kubo, Rainbow) continues to work.

References

• CC #771
• Closes #772

[codecov[bot]]

Codecov Report

:x: Patch coverage is 66.66667% with 6 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 60.76%. Comparing base (4c0aa3a) to head (b3238d9).

Files with missing lines	Patch %	Lines
gateway/dns.go	66.66%	4 Missing and 2 partials :warning:

@@            Coverage Diff             @@
##             main    #1044      +/-   ##
==========================================
- Coverage   60.76%   60.76%   -0.01%     
==========================================
  Files         268      268              
  Lines       33593    33595       +2     
==========================================
- Hits        20414    20413       -1     
+ Misses      11508    11507       -1     
- Partials     1671     1675       +4     

Files with missing lines	Coverage Δ
gateway/dns.go	57.44% <66.66%> (-2.56%)	:arrow_down:

... and 13 files with indirect coverage changes

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