Include hidden arguments in shell completion scripts

[pintaste]

Summary

Arguments marked with .hidden visibility are currently excluded from shell completion scripts, along with .private arguments. However, hidden arguments are still part of the public API — they're just deprioritized in help output. Only .private arguments should be excluded from completions.

This PR updates the completion script generation so that:

• Hidden arguments (.hidden) are included in tab completions (bash, fish, zsh)
• Private arguments (.private) remain excluded from completions
• Help output is unchanged — hidden args still don't appear in --help

Changes

• DumpHelpGenerator.swift: Added includeHiddenArguments parameter to ToolInfoV0.init(commandStack:) and CommandInfoV0.init(commandStack:). When true, sets shouldDisplay = true for .hidden arguments (in addition to .default ones), so completion scripts include them.
• CompletionsGenerator.swift: Pass includeHiddenArguments: true when building ToolInfoV0 for completion script generation.
• Updated existing completion snapshots to include the hidden --verbose flag from the Base test command and the hidden --test-* flags from the math example.
• Added testHiddenAndPrivate_Bash/Fish/Zsh test cases verifying hidden args appear in completions and private args don't.

Testing

swift test --filter "CompletionScriptTests"

All 9 completion tests pass (including 3 new ones).

Fixes #733

[kiwigitops]

I think there are still generated-script regressions in the snapshots after the separator fix. Several bash and fish fixtures now contain actual line breaks inside shell constructs that previously emitted escaped newline sequences, for example in testMathBashCompletionScript().bash, testBase_Bash().bash, and the printf/file-completion expression in testBase_Fish().fish.

There is also an escaping change in shellEscapeForSingleQuotedString: the replacement now drops one backslash from the single-quote escape sequence. That seems unrelated to including hidden arguments and looks like it can under-escape generated completion descriptions such as the escaped-command snapshots.

[kiwigitops]

Thanks, the latest commit restores the $'\n' snapshot sequences and the shellEscapeForSingleQuotedString escape. That addresses the generated-script regression I was worried about.