(PE-45366) Add pe_run_in_parallel option, serial by default on macOS

[Magisus]

What's this PR do?

Adds a pe_run_in_parallel switch controlling whether beaker-pe's PE install/upgrade steps fork a child process per host or run serially, and changes the default to serial on macOS.

Several install/upgrade steps act on many hosts by fork()ing a child per host, each with its own Net::SSH connection. On macOS, forking a Ruby process that has already used Net::SSH deadlocks the child, so those steps hang until beaker's watchdog kills them after 1800s (RuntimeError: Child process ran longer than timeout of 1800). This never reproduces on Linux/CI.

A new private helper pe_run_in_parallel? on PEUtils resolves a single boolean, precedence highest first:

1. BEAKER_PE_RUN_IN_PARALLEL env var (1/true/yes = parallel, 0/false/no = serial)
2. options[:pe_run_in_parallel] option
3. Default: false on macOS, true on every other platform

The 5 hardcoded :run_in_parallel => true literals in lib/beaker-pe/install/pe_utils.rb now call the helper. Mac developers get serial execution out of the box, so the deadlock no longer bites; a developer on any platform can override the default either direction via the option or env var.

Should any of this be tested outside the normal PR CI cycle?

CI runs on Linux, where the default resolves to true, so behavior is byte-for-byte unchanged there. The macOS serial path is exercised by unit specs (the platform default, option, and env var all resolve to false and reach the call sites); the underlying deadlock itself only reproduces on a Mac and isn't covered by CI.

Any background context you want to provide?

• The default is derived from RUBY_PLATFORM (the machine running beaker, which is the process that forks), matching /darwin/.
• The spec suite is made hermetic against both a developer's exported BEAKER_PE_RUN_IN_PARALLEL and being run on a Mac, by stubbing the env var and RUBY_PLATFORM for every example. Pre-existing :run_in_parallel => true specs still pass because they run against the pinned non-macOS platform.
• README gains a ## Configuration section documenting the option, env var, precedence, and the macOS default.
• Labeled enhancement for the changelog generator (feature → minor version bump).

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