WIP: Add test coverage for `ConnectStream` #298

ref: #183

Opening this draft to make it visible to more reviewers. Quoting my comment in the issue as well:

I've added tests for four scenarios:

1. Attempting to use an fd associated with a server that has already been disconnected results in a crash with: "IPC client method called after disconnect".
2. Attempting to use an fd associated with a server that disconnects immediately after the first recv(), the outcome will either be crashing with "IPC client method called after disconnect" or "IPC client method call interrupted by disconnect." This behavior seems to depend on whether the recv() call successfully reads the entirety of what is likely Capnp’s bootstrap request or only a portion of it.
3. Attempting to use an fd associated with a server that disconnects after a second recv(), the process crashes with "IPC client method call interrupted by disconnect."
4. Attempting to use an fd associated with a server that indefinitely calls recv() in a while loop results in the test getting stuck at this line:

https://github.com/bitcoin-core/libmultiprocess/blob/8412fcdc659e1379f9b4dea896c26bc1c5f3afa8/include/mp/proxy-types.h#L754

It’s important to note that all of these cases call FooInterface::initThreadMap() to invoke clientInvoke(), as ConnectStream() on its own won't trigger any libmultiprocess request if there's not a construct() method available in the Init interface.

I'm looking for more ideas for testing ConnectStream.

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Possible typos and grammar issues:

• Cap N' Proto -> Cap'n Proto [the project name is misspelled]

^{2026-06-24 03:54:02}