fix(plugins): keep root help responsive

[stephenlclarke]

Type of Change

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

Motivation and Context

Root help currently performs daemon-backed plugin discovery before it can print. If the API service is registered but not replying, container --help, container help, and bare container can appear to hang.

Related to #1459. Supersedes #1838, whose fork branch was deleted after the PR was closed.

This PR is intentionally stacked on #1862:

• The first commit imports fix: reply to unknown XPC routes and make request timeout cancellable #1862 unchanged on current main, preserving its original author and implementation in a standalone commit.
• The second commit adds deterministic cancellation, late-reply, client-reuse, and unknown-route tests for fix: reply to unknown XPC routes and make request timeout cancellable #1862.
• The final commit is the intended help/plugin delta.
• If fix: reply to unknown XPC routes and make request timeout cancellable #1862 lands first, the imported commit can be dropped during rebase and this PR reduces to tests plus the help-specific deadline.

What Changed

• Makes XPC response timeouts and caller cancellation return promptly through the cancellable request handling from fix: reply to unknown XPC routes and make request timeout cancellable #1862.
• Returns invalidArgument for unknown XPC routes instead of leaving clients waiting.
• Uses a one-second API health deadline for root help plugin discovery.
• Preserves the existing ten-second health deadline for real plugin dispatch.
• Falls back to built-in help when the API service is unavailable.
• Covers container --help, container help, and bare container in integration tests.
• Avoids process-global stdout redirection in unit tests.

Testing

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

Validation:

• make all
• make test
• make swift-fmt-check
• swift test --disable-automatic-resolution --filter 'ApplicationPluginDiscoveryTests|XPCClientTests': 8 tests passed
• Focused coverage: XPCReplyBox.swift reached 100% line, function, and region coverage; every new help-policy branch was executed
• Targeted CLI integration: container --help, container help, bare container, and container --debug help passed
• Full integration at width 4 exercised the new help paths successfully

The full integration matrix also exposed repository-baseline failures unrelated to this diff:

• Three serial .dockerignore tests failed with four assertions.

• The same three tests fail identically from a clean worktree at unmodified Apple main (5f277a9).

• The default 18-way run also saturated host forwarding; all 32 tests in the affected run-command suite passed when rerun serially.

• git diff --check origin/main..HEAD