[Bug]: New `env` bootstrap parameter breaks XPC API compatibility.

[jglogan]

I have done the following

• I have searched the existing issues
• If possible, I've reproduced the issue using the 'main' branch of this project

Steps to reproduce

I tried running a plugin that used an older version of the client against the main branch, and saw this error:

Error: failed to bootstrap container (cause: "invalidArgument: "env cannot be empty"")

Problem description

Older clients that don't send env with bootstrap should use a default of [:].

There are a couple of other nits:

• env as a func parameter and XPC dictionary key is ambiguous when we hav --env for container create/run and ENV in Dockerfiles, both for the immutable container environment. Since these are special-case variables dynamically computed at start, it makes sense to use dynamicEnv.
• [String:String]? is overcomplicated, in almost all cases we should be using Type? for 0..1 cardinality and [Type] for 0..* cardinality.

Environment

- OS: 26
- Xcode: 26
- Container: main

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