[Release-1.27] Move proxy dialer out of init() and fix crash

[dereknola]

Proposed Changes

Backport #9219 to release-1.27

• Fixes issue where proxy support only honored server address via K3S_URL, not CLI or config.
• Fixes crash when agent proxy is enabled, but proxy env vars do not return a proxy URL for the server address (server URL is in NO_PROXY list).
• Adds tests

Types of Changes

Verification

Testing

Linked Issues

#9114

User-Facing Change

Further Comments

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (e9b2e59) 46.68% compared to head (ffbaf80) 45.80%.
Report is 21 commits behind head on release-1.27.

Files	Patch %	Lines
pkg/agent/proxy/apiproxy.go	0.00%	2 Missing and 1 partial ⚠️
pkg/agent/loadbalancer/servers.go	88.88%	2 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-1.27    #9220      +/-   ##
================================================
- Coverage         46.68%   45.80%   -0.89%     
================================================
  Files               148      148              
  Lines             15861    15869       +8     
================================================
- Hits               7405     7269     -136     
- Misses             7251     7433     +182     
+ Partials           1205     1167      -38     

Flag	Coverage Δ
inttests	43.22% <14.28%> (-1.03%)	⬇️
unittests	18.54% <88.88%> (+0.22%)	⬆️

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