Apply --ulimit in container exec and machine run

[rishabhsai]

Type of Change

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

Motivation and Context

container exec and container machine run embed Flags.Process, so both print --ulimit <limit> in their help and accept the flag without error, but neither ever reads processFlags.ulimits; the only reader is Parser.process on the run/create path. Exec therefore keeps whatever rlimits the init process was started with and the value you passed is dropped with no warning, even though the guest already applies per-process rlimits on exec via App.setRLimits(rlimits: process.rlimits) in containerization's vmexec/ExecCommand.swift. #1129 added --ulimit to Flags.Process and wired it into run and create only, which is where the gap came from. This sets config.rlimits on exec when --ulimit is given, leaving the init process limits in place when it is not, and passes rlimits: into the ProcessConfiguration that machine run builds.

Fixes #2266

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

swift build --product container succeeds and .build/debug/container exec --help | grep ulimit still lists the flag, and make swift-fmt leaves the diff unchanged. The new testExecUlimitNofile in TestCLIExecCommand is compile-verified only: the integration suite needs an installed CLI and a running container system, which I could not run locally, so every test in that suite fails at fixture setup with binaryNotFound on my machine.