fix(plugins): bound discovery when apiserver stalls

[stephenlclarke]

Type of Change

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

Motivation and Context

Plugin discovery can hang before root help or plugin dispatch completes when com.apple.container.apiserver is registered but not answering XPC requests. This affects paths such as container --help, container help, bare container, and container compose ....

Addresses #1459.

Related to #995, but this PR intentionally keeps a narrower scope:

• fix(xpc): prevent startup hangs with proper timeout handling (#561) #995 also changes apiserver startup/default-network behavior and adds broader NetworkClient timeout policy.
• This PR only bounds plugin discovery/help and fixes the shared XPCClient.send(... responseTimeout:) timeout primitive.
• This PR keeps XPC clients reusable after a timed-out request; late replies are ignored by a single-resume gate instead of closing the connection.
• This PR adds deterministic command-health and anonymous-XPC regression tests.

Testing

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

Validation:

• swift test --disable-automatic-resolution --enable-code-coverage --filter 'ApplicationHealthTests|XPCClientTests|HelpCommandTests'
• make swift-fmt-check
• git diff --check

The focused coverage suite covers the new timeout/race behavior, root/default/help command paths, XPC timeout return, nil-route timeout messaging, normal XPC replies, and client reuse after timeout.

[stephenlclarke]

Superseded by the current open, ready-for-review replacement #1935. The replacement imports the preferred XPC cancellation work from #1862 in its own commit, adds deterministic coverage, and keeps the remaining root-help policy delta focused.