Avoid Locust master port collisions in virtual benchmarks

[Copilot]

Benchmark Virtual could fail when Locust selected a port available on loopback but then bound its master listener on all interfaces. Another process bound on a non-loopback interface caused Locust startup to fail.

• Locust master binding
  • Bind the master RPC listener explicitly to localhost.
  • Select the port using the same loopback address scope.

master_host = "localhost"
master_port = infra.net.probably_free_local_port(master_host)
cmd += ["--master-bind-host", master_host, ...]

[Copilot]

🟢 Approval recommended

The change is small, localized, and directly addresses the stated failure mode by aligning port selection and bind scope to loopback.

Pull request overview

This PR hardens the virtual benchmark Locust invocation to avoid master port collisions caused by selecting a port on loopback but then binding the master listener on all interfaces.

Changes:

• Introduces an explicit master_host = "localhost" for Locust master binding.
• Selects master_port using the same loopback scope and passes --master-bind-host to ensure Locust binds only on loopback.

Custom instructions used

• None (no repository instruction files from .github/copilot-instructions.md / .github/instructions/ were explicitly loaded via tools during this review).

File summaries

File	Description
tests/infra/locust_benchmark.py	Bind Locust master RPC listener explicitly to loopback and select the port using the same address scope to prevent interface-scope collisions.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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