Run string-form dev container lifecycle commands in /bin/sh -c

[rabindra789]

Objective

Fixes #60228.

String-form dev container lifecycle commands are currently executed by
splitting the command on spaces and running the first token directly.
This means shell features such as variable expansion, pipes,
redirection, and command chaining do not work.

According to the Dev Container specification, string-form lifecycle
commands should run through /bin/sh -c, while array-form commands
should continue to execute directly.

Solution

• Added LifecycleScript::shell_command() to wrap string-form commands in /bin/sh -c.
• Updated string deserialization to use shell_command() instead of splitting on spaces.
• Kept array-form commands unchanged.

Testing

• Ran cargo test -p dev_container (107 tests passed).
• Added a regression test to verify string-form commands run through /bin/sh -c.
• Added a regression test to verify array-form commands still execute directly.
• Updated the existing deserialization tests to match the expected behavior.

Self-Review Checklist

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content adheres to Zed's UI standards
• Tests cover the new behavior
• Performance impact has been considered and is acceptable

---

Release Notes:

• Fixed string-form dev container lifecycle commands so they execute through /bin/sh -c as required by the Dev Container specification.

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[rabindra789]

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[DerekParks]

@rabindra789
Thanks for putting this up!