Overview

Let a built-in command opt in, at registration, to a predicate that says "an extension replaces me". When the predicate is true at startup, the command is not constructed or registered, so an extension-provided command of the same name can be registered instead. No existing command opts in as part of this ticket, so there is no behavior change.

Part of SPM-4830 (Support Commands in the Extensions API). The search index management commands will opt in later, when extension or built-in search index commands are selected at startup.

Scope of Work

• CommandConstructionPlan::EntryBuilder gains replaceableByExtension(std::function<bool()>), stored on the plan Entry.
• CommandConstructionPlan::execute skips any entry whose replacement predicate returns true.

Acceptance Criteria

• A command whose replacement predicate is true is missing from the registry.
• A command whose predicate is false, or that never opted in, is registered as before.

Example Test Cases

• Command construction-plan unit tests covering a predicate that is true, one that is false, and a command that never opted in, for both shard and router registries.

Technical Notes

• This adds API to commands.h that has no production caller until the search index commands opt in. If server-programmability reviewers prefer, merge this with that work instead.

Depends on: none