test: add test for parameter expansion with command substitution ${VAR:-$(cmd)}

[chaliy]

This PR adds a comprehensive test for the parameter expansion behavior with command substitution in default values.

Test Coverage

• Unset variable: ${VAR:-$(echo hello)} runs command substitution
• Empty variable: ${VAR:-$(echo hello)} runs command substitution (:- means unset OR empty)
• Set variable: ${VAR:-$(echo hello)} does NOT run command substitution
• Without colon: ${VAR-$(echo hello)} only runs for unset, not empty

The implementation already correctly handles these cases, matching bash behavior exactly.

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	bashkit	f0a5ee8	Commit Preview URL Branch Preview URL	Oct 12 2026, 01:31 AM