Fix isolation error wording and log-level rationale from #5794 review

[aponcedeleonch]

Summary

#5794 (merged) reconciled network isolation with network mode. rdimitrov's
post-merge review left two follow-up comments this PR addresses:

• The fail-fast error for --isolate-network=true conflicting with a
  non-bridge network mode assumed the mode always came from --network,
  telling users to drop --network host even when the mode actually came
  from a --permission-profile file (where there's no --network flag to
  drop). The error now names the resolved mode directly instead of assuming
  a specific flag.
• Clarified in degradeNetworkIsolation why the none and host/custom
  degrade paths intentionally log at different levels: none is DEBUG
  (redundant, not a security reduction), while host/custom stays at WARN
  even on read paths (thv list, status, export) since it means the
  user's --isolate-network request is being silently ignored.

Type of change

• Bug fix

Test plan

• Unit tests (task test)
• Linting (task lint-fix)

Updated TestRunConfigBuilder_NetworkIsolationReconciliation in
pkg/runner/config_builder_test.go to assert the error names the resolved
mode instead of an assumed --network flag.

Does this introduce a user-facing change?

Yes. The fail-fast error message when --isolate-network=true conflicts
with a non-bridge network mode now names the resolved network mode
directly (e.g. "host") instead of assuming it came from --network.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.23%. Comparing base (e14bb31) to head (3bcf1e8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5853      +/-   ##
==========================================
+ Coverage   71.16%   71.23%   +0.06%     
==========================================
  Files         689      689              
  Lines       70110    70110              
==========================================
+ Hits        49896    49941      +45     
+ Misses      16595    16533      -62     
- Partials     3619     3636      +17     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.