fix: handle net.Listen error in gRPC ExternalScaler goroutine (#323)

[abhaygoudannavar]

What this PR does

Fixes a nil-pointer panic in the gRPC ExternalScaler server goroutine caused by a discarded net.Listen error.

Problem

In main.go, the error returned by net.Listen was silently discarded:

lis, _ := net.Listen("tcp", scaleServerAddr)

When Listen fails (e.g. port conflict, permission denied), lis is nil. The subsequent call to grpcServer.Serve(nil) dereferences the nil listener and panics, crashing the controller-manager with no diagnostic log output.

Fix

• Check the net.Listen error and log it with the bind address before exiting.
• Improve the Serve error message to distinguish listen vs. serve failures.

Fixes #323

[kruise-bot]

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[kruise-bot]

Welcome @abhaygoudannavar! It looks like this is your first PR to openkruise/kruise-game 🎉

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 40.53%. Comparing base (0974487) to head (d2f364b).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #324   +/-   ##
=======================================
  Coverage   40.53%   40.53%           
=======================================
  Files         112      112           
  Lines       12547    12547           
=======================================
  Hits         5086     5086           
  Misses       7051     7051           
  Partials      410      410           

Flag	Coverage Δ
unittests	40.53% <ø> (ø)

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