SemanticCodeSearch: Handle no collection record

Context

If there is no "current" Ai::ActiveContext::Collection record in an instance, we should make sure that the semantic_code_search tool is not available.

Optional additional consideration

Consider whether the Ai::ActiveContext::Queries::Code should return a failed Result object for the "no collection record" error. Currently, it throws an exception for the reasons stated in !208905 (comment 2827429735), in particular:

I see a "no collection record" error as a server issue (like a 500). The Query should raise an exception because embeddings indexing is not enabled for the instance.

• This is as opposed to a "project has no embeddings", which is an issue with the project that the client provided (so it's more a 400 error)

Proposal

Add a check in API::Mcp::Handlers::ListTools#exclude_tool? to exclude the semantic_code_search if ::Ai::ActiveContext::Collections::Code.collection_record is nil.