chore(search): remove unused names index client code

[gbirman]

Stacked on #2694.

Summary

• The names OpenSearch index (3M docs / 2.5GB in prod) has no writers: upsert_entity_name exists but has zero callers in the codebase. The only code still touching it is defensive cleanup in chat/document/email removal paths, which delete from an index nothing writes to.
• Drop the orphaned upsert_entity_name + UpsertEntityNameArgs and all four delete_entity_name* functions (+ their OpensearchClient wrappers).
• Remove the defensive delete_entity_name* calls from search_processing_service::process::{chat, document::mod, email::remove, user}.
• Remove SearchIndex::Names variant — with these deletions, it has no references.

Once this deploys, the prod names index can be deleted out of band (same pattern as the projects index).

The unit test asserting OpenSearchEntityType::index_name() matches SearchIndex::from(variant).as_ref() (added in #2694) still passes — Names wasn't part of that invariant.

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⚙️ Run configuration

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Run ID: c7154c6d-8d25-4e07-acce-8cd42e1c1d3d

📥 Commits

Reviewing files that changed from the base of the PR and between ff0d828 and 97b79c5.

📒 Files selected for processing (12)

• infra/stacks/opensearch/helpers/scripts/delete_user_entities.ts
• rust/cloud-storage/models_opensearch/src/lib.rs
• rust/cloud-storage/opensearch_client/src/delete/mod.rs
• rust/cloud-storage/opensearch_client/src/delete/name.rs
• rust/cloud-storage/opensearch_client/src/lib.rs
• rust/cloud-storage/opensearch_client/src/name.rs
• rust/cloud-storage/opensearch_client/src/upsert/mod.rs
• rust/cloud-storage/opensearch_client/src/upsert/name.rs
• rust/cloud-storage/search_processing_service/src/process/chat.rs
• rust/cloud-storage/search_processing_service/src/process/document/mod.rs
• rust/cloud-storage/search_processing_service/src/process/email/remove.rs
• rust/cloud-storage/search_processing_service/src/process/user.rs

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