Validate XPC peers with xpc_connection_get_euid instead of the non-public audit-token API

[devops-thiago]

Fixes #2087.

Type of Change

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

Motivation and Context

#896 added same-EUID validation of XPC peers by reading each message's audit token via xpc_dictionary_get_audit_token. That symbol is not in the public XPC headers — the CAuditToken target exists solely to re-declare its prototype — and App Store Connect rejects binaries that reference it (see #2087 for the verbatim rejection), which blocks anyone redistributing these executables inside a reviewed app.

Description

The check's purpose (client EUID == server EUID) is served by public API: for a launchd Mach service, every peer arrives on its own connection, so the connection's credentials and the message sender's are the same identity, and xpc_connection_get_euid is the documented way to read them.

• XPCServer.handleMessage reads the peer EUID from the connection.
• The CAuditToken shim target is deleted along with its libbsm link dependency; Package.swift references removed.

Net: 9 insertions, 52 deletions, no behavior change to the enforcement.

Testing

• swift build clean on this branch at abff418; nm -u container-apiserver shows zero audit_token references afterwards.
• We run this change in production in a GUI embedder of this engine: the same-user gate demonstrably still admits legitimate peers (an end-to-end suite of ~95 checks crosses it on every request) — and the previously rejected App Store submission passed the non-public-API scan with it.