header_rewrite: reject bad run-plugin at config load

[zwoop]

A run-plugin whose target plugin failed to load left a null instance that tripped a release assert and aborted the server on the first request. Propagate the load failure as an exception so the rule is rejected at config load time (a reload simply keeps the running config), and guard exec() so a stray bad rule can never abort the process.

[Copilot]

Pull request overview

This PR hardens the header_rewrite plugin’s run-plugin operator so that failures to load the target remap plugin are rejected during configuration load/reload rather than causing a TSReleaseAssert abort on first request. It also adds an AuTest gold test to prevent regressions and validate both cold-start and reload behavior.

Changes:

• Convert OperatorRunPlugin::initialize() load failures into exceptions, so bad run-plugin rules are rejected during config parsing.
• Catch operator initialization exceptions in RuleSet::add_operator() and fail the ruleset cleanly with an error log.
• Add a gold test covering both startup rejection and reload rejection while keeping the running config active.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/gold_tests/pluginTest/header_rewrite/header_rewrite_bad_run_plugin.test.py	Adds coverage ensuring a bad run-plugin is rejected at config load/reload and never crashes request processing.
plugins/header_rewrite/ruleset.cc	Catches Operator::initialize() exceptions so operator init failures reject the config cleanly.
plugins/header_rewrite/operators.cc	Makes run-plugin initialization throw on load failures and guards exec() against a null plugin instance.

[zwoop]

[approve ci]