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

Implement proposed follow-ups on dicussion #35084 (review).

• Add unit test for checkSpawned().
• Make checkSpawned() return std::optionalmp::SocketId.
• Rename -ipcfd to -ipcchild.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35887.

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Reviews

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LLM Linter (✨ experimental)

Possible places where comparison-specific test macros should replace generic comparisons:

• [src/ipc/test/ipc_tests.cpp] BOOST_CHECK_THROW(process->checkSpawned(3, argv), std::runtime_error); -> consider BOOST_CHECK_EXCEPTION(..., std::runtime_error, HasReason("...")) to verify the specific failure message instead of only the generic exception type.

^{2026-08-04 20:47:50}