Stop unknown coprocessor stats / no output from shell commands.

[Juniormunk]

Description

Join the threads and wait for them to finish reading the input/error streams before returning.

Meta

Merge checklist:

• Pull Request title is short, imperative summary of proposed changes
• The description documents the what and why
• If this PR changes behavior or adds a feature, user documentation is updated
• If this PR touches photon-serde, all messages have been regenerated and hashes have not changed unexpectedly
• If this PR touches configuration, this is backwards compatible with settings back to v2024.3.1
• If this PR addresses a bug, a regression test for it is added

[mcm001]

TY! I think we should probably replace isOutputCompleted and isErrorCompleted with this same join (or set a timeout, as appropriate), or delete these if they're redundant, as well. For example, this already waits for completion, so we don't need that extra wait anymore.

[mcm001]

Looks like it'll fix a race