Replace tuples with better suited data structures for server references (#86294)

This PR introduces the `ServerReferenceExport` struct to replace `(Ident, Atom, Atom)` tuples for tracking exported server references. It also replaces the `Vec<(Atom, Atom)>` `export_actions` with an `FxIndexMap` for `reference_ids_by_export_name`, enabling more efficient lookups in upstack PRs. There are no functional changes.

Author: unstubbable