Fix forwarded process signal encoding

[vincent-peng]

Summary

• Encode forwarded process signals as signal names instead of integer XPC values.
• Preserve Darwin-to-Linux signal semantics for foreground signal forwarding.
• Add coverage for the kill request payload and forwarded INT, TERM, WINCH, USR1, and USR2 mappings.

Root cause

Foreground non-tty signal forwarding sent .signal as an integer, but the API server reads that field as a string before forwarding it to the runtime. That made forwarded resize signals fail with missing signal in xpc message. A numeric-only fix would also risk mis-mapping Darwin USR1/USR2 values to Linux.

Fixes #1747

Validation

• swift test --filter ContainerAPIClientTests.ClientProcessTests
• swift test --filter ContainerAPIClientTests
• swift build --target ContainerAPIClient