Fix XPC server crash on fire-and-forget messages

[1PoPTRoN]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

Fixes #2281.

XPCMessage.reply() force-unwrapped xpc_dictionary_create_reply(), which returns NULL for any message sent fire-and-forget via xpc_connection_send_message. Every path in XPCServer.handleMessage — all four error paths through replyWithError plus every success path built by the route handlers (88 reply() call sites across 13 files) — hits this, so one one-way dictionary from any same-uid process traps container-apiserver or any helper (container-runtime-linux, container-network-vmnet, container-core-images, machine-apiserver) with Fatal error: Unexpectedly found nil at XPCMessage.swift:60.

The fix keeps the reply() signature (no caller churn) but makes it non-trapping with an empty-dictionary fallback, adds a canReply check, and drops no-reply-context input in handleMessage/replyWithError after logging. Legitimate _with_reply traffic takes the identical code path as before.

Testing

• Tested locally
• Added/updated tests
• Added/updated docs (no user-facing behavior change for legitimate traffic, so no doc updates)

Verification done on main @ 57f0b93, macOS 27.0, CommandLineTools Swift 6.4:

• Live repro (anonymous XPC pair + one-way send, then reply() on the received dict): traps pre-fix (exit 133/SIGTRAP at XPCMessage.swift:60), passes post-fix with a usable fallback dictionary.
• New ContainerXPCTests suite (4 tests: no-reply-context detection, no-trap regression, round-trip, live connection-pair repro). Note: this box has CLT only, so the swift-testing suite itself was validated via equivalent standalone harnesses linked against the built modules — please let CI run ContainerXPCTests under Xcode.
• Full swift build green; swift format lint --strict clean on all touched files.

[1PoPTRoN]

Hi @jglogan @katiewasnothere — I'm new around here and this is my first contribution, so please point me at anything I'm doing wrong. This one crashes container-apiserver (and every helper) on a single one-way dictionary, so I wanted eyes on it early.

I kept reply() non-failable with a fallback to avoid churning all 88 call sites, but I'm still learning the conventions here — happy to rework it however you'd like.