Stabilize installer cancellation readiness

[Widthdom]

Summary

• wait for the installer PID file to contain a complete positive integer before cancelling the hung installer
• use a bounded dedicated waiter so parallel load cannot expose the empty file created by shell redirection
• document the test contract in English and Japanese and add a changelog fragment

Root cause

The previous fixed 100 ms cancellation could interrupt the shell after it opened the redirected PID file but before it wrote $$. The test then parsed an empty string as an integer.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• targeted installer cancellation test on net8.0 and net9.0
• exact issue filter: 859/859 tests passed
• full solution test run: one unrelated, pass-on-isolated-rerun failure per target framework; tracked in MCP timed-out concurrency lease test can null-dereference under full-suite load #4807 and Full-suite ConsoleCapture timeout test can restore a different writer under load #4808
• dotnet run --project tools/CodeIndex.Changelog -- check
• make lint
• two Codex adversarial review rounds: no blocking or actionable findings

Documentation and changelog

• updated TESTING_GUIDE.md in English and Japanese
• added changelog.d/unreleased/4801.fixed.md

Follow-up issues

• MCP timed-out concurrency lease test can null-dereference under full-suite load #4807
• Full-suite ConsoleCapture timeout test can restore a different writer under load #4808
• HOME shorthand log-path test disagrees with sandboxed HOME #4810

Fixes #4801