[cherry-pick] test: fix resolver tunnel port allocation and bind errors

[vs-code-engineering]

Cherry-pick of #336996 from main.

Summary

Narrowed to the test resolver only. The browser-tools HTTP fixture is handled independently by #336988; this PR no longer changes that test.

• When the requested local port equals the remote port (the test resolver runs both on the same machine), let the OS allocate a free local port instead of assuming remotePort + 1 is available. The adjacent port can be occupied or reserved on Windows, and remote port 65535 would otherwise produce an invalid local port.
• Reject tunnel creation on a server bind error instead of leaving the promise pending.
• Preserve explicitly requested distinct local ports and existing privileged-port behavior.

This addresses the resolver failure behind the rollback in #336381. It is not an additional failure established in Azure build 475409, and #336988 does not require this change because it uses the browser remote proxy without explicitly opening a tunnel.

Validation

• After narrowing: both test extensions compile with zero errors; scoped hygiene (including ESLint) and git diff --check pass.
• Direct compiled-resolver probes passed: remote port 65535 allocates a valid local port; an occupied explicitly requested local port rejects with EADDRINUSE.
• The resolver implementation is unchanged from ec5a082570d, validated by Azure build 475477: succeeded, with all Electron and Remote integration jobs passing on Windows x64, Linux x64, and macOS ARM64. That revision still included the now-removed HTTP fixture, which explicitly exercised tunnel creation in remote tests.
• The scope-only follow-up commit has not been rerun in Azure.

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused resolver changes address the reported failures without unresolved issues.

Review effort: Balanced
Findings: None

What changed in this PR

Improves test-resolver tunnel reliability across platforms.

Changes:

• Lets the OS allocate a port when local and remote ports match.
• Rejects tunnel creation on bind errors.

File	Description
extensions/​vscode-test-resolver/​src/​extension.ts	Handles tunnel port allocation and bind failures.

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