feat: default create_index to create-only - #4124

This PR changes create_index to be create-only by default in LanceDB. Before this change, omitting replace could rebuild an existing index, which made duplicate create calls surprising when callers only meant to create a new index. The new default is replace=false; callers that want to rebuild an existing index must pass replace=true explicitly. This is an intentional breaking behavior change for duplicate index creation.

This was accomplished with the following changes:

• IndexBuilder::new now initializes replace to false, so Rust/local index creation defaults to create-only.
• Remote index submission in RemoteTable::submit_create_index always serializes replace, preserving both the new default and explicit replace=true.
• Python sync, async, and remote wrappers now default replace to False and pass that value through consistently.
• Node API docs and duplicate-index tests now reflect create-only default behavior.
• Remote-only async job helpers in rust/lancedb/src/job.rs are gated behind the remote feature so default-feature builds compile cleanly.

Testing

• Updated Rust tests for default async duplicate-index failures and remote request serialization.
• Updated Python index and wrapper tests for default create-only behavior and explicit replacement.
• Updated Node duplicate-index test expectations.