fix(extensionserver): validate rate limit service address

[0YHR0]

Description
Reject malformed quotaRateLimitServiceAddr values instead of treating every net.SplitHostPort error as a missing port. Hostnames and bare IPv6 literals continue to use the default port, while malformed addresses now fail fast during extension server startup.

Fixes #2362

Testing

• go test ./internal/extensionserver -count=1

[0YHR0]

@missBerg pls take a look, thx :)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.04%. Comparing base (25afc17) to head (4020347).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2363      +/-   ##
==========================================
- Coverage   85.05%   85.04%   -0.02%     
==========================================
  Files         159      159              
  Lines       22645    22649       +4     
==========================================
+ Hits        19261    19262       +1     
- Misses       2217     2219       +2     
- Partials     1167     1168       +1     

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