#35887 ipc: use std::optional for checkSpawned(), add tests and rename arg -ipcfd to -ipcchild

• Make internal IPC argument names platform-neutral and modernize process checking interfaces
• Add unit test coverage for IPC child process argument parsing edge cases

Implements follow-up suggestions from the cross-platform IPC support PR (#35084). It changes `ipc::Process::checkSpawned()` to return `std::optional<mp::SocketId>` instead of a boolean with an out-parameter, renames the internal argument `-ipcfd` to `-ipcchild` to reflect non-FD platforms like Windows, and adds unit tests for process argument parsing.

Problem: The `checkSpawned()` method used a pre-C++17 signature with an out-parameter, lacked unit test coverage for argument parsing edge cases, and used the name `-ipcfd` which was misleading on platforms like Windows that use named pipes instead of file descriptors.

P3 · cleanup

• P3 because it is minor cleanup and test coverage for multiprocess support
• Improves argument accuracy on non-UNIX platforms without fixing active bugs

Follows up on merged cross-platform IPC work (#35084) by adopting modern C++ patterns, improving argument naming on Windows, and adding unit tests. It is worthwhile cleanup and test coverage for multiprocess support, but is reasonably deferrable and fixes no active bugs.

Membership: Directly touches IPC interface headers, child process spawning helpers, and IPC test harnesses in src/ipc/.

Factors: security/stability 0, bug 0, performance 0, user value 0, leverage 1

• Ready for review, clean, and passing CI with all feedback addressed

The PR is clean, passing CI, and has addressed reviewer requests.

Author status: active

Resolved concerns:

• jeanpablojp noted a missing test case covering three arguments where the second is not `-ipcchild`; author added the test case in an updated push.

• Strong support for completing the requested follow-ups from earlier IPC work
• Verified by testing with IPC enabled and checking argument handling (jeanpablojp)
• Approved commit structure and code changes (enirox001)

Strong: two reviewers verified and ACKed the implementation of the requested follow-ups

Multiple contributors reviewed and tested the changes, confirming that the follow-ups requested during #35084 review are satisfied, with all review feedback resolved.

• jeanpablojp confirmed testing with IPC enabled and verified follow-up implementation from #35084, subsequently ACKing head.
• enirox001 utACKed the commit structure and implementation.

Review verdicts (DrahtBot): 2

• ACK: jeanpablojp, enirox001

2 lines under test/bench/ci.

• src/ipc/test/ipc_tests.cpp +41/-0
• src/ipc/process.cpp +16/-16
• src/ipc/interfaces.cpp +4/-3
• src/ipc/process.h +4/-3
• src/bitcoin.cpp +1/-1
• test/lint/check-doc.py +1/-1

This PR refactors `ipc::Process::checkSpawned()` to return `std::optional<mp::SocketId>`, renames `-ipcfd` to `-ipcchild`, and adds unit tests in `ipc_tests.cpp`. It solves minor technical debt and naming inaccuracy on Windows left over from #35084 as requested by reviewers. The change has strong support with tested ACKs and no unresolved concerns. It is self-contained with no remaining dependencies.