Fix executable help hints in suggestions errors

[Widthdom]

Summary

• replace the literal usage-hint placeholder with a catalog-backed, command-aware builder and a safe global fallback
• direct suggestions human and JSON usage errors to cdidx suggestions --help while preserving the existing error, usage, and exit-code contracts
• add focused regression coverage and bilingual testing/changelog documentation

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -f net8.0 -p:UseSharedCompilation=false --filter 'FullyQualifiedName~CommandErrorWriterTests|FullyQualifiedName~ProgramCliTests.Suggestions_UsageErrorsUseExecutableHelpHints_Issue5196' (23 passed)
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false (0 warnings, 0 errors)
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-build (net8: 12,041 passed / 9 skipped; net9: 11,556 passed / 431 skipped; 0 failed)
• local cdidx root and workspace index health checks

Documentation and changelog

• Updated the English and Japanese sections of TESTING_GUIDE.md.
• Added changelog.d/unreleased/5196.fixed.md.
• No README or user-guide update was needed because no command, option, or user workflow changed; this corrects an invalid diagnostic hint.

Review

• codex review --base origin/main: no blocking/actionable findings.

Follow-up candidates

• None.

Fixes #5196