Simplify FabricMountingManager::executeMount (#57154)

Summary:

`FabricMountingManager::executeMount` (CCN 59) dispatched a large per-mutation-type `switch` with repeated buffer-append boilerplate. This is a pure, behavior-preserving refactor: the five mutation-type case bodies are extracted into file-local `static` helpers backed by a small `MountItemBuffers` struct (holding `maintainMutationOrder` plus references to the output vectors, with an `orderedOr()` helper reproducing the exact `maintainMutationOrder ? common : bucket` selection), and the seven repeated empty-guard + per-item loops collapse into one templated batch writer. Ordering and side effects are preserved exactly (including the Update-case event-emitter quirk and Delete-batch-last ordering). Anonymous-namespace helpers only; no header or public API change.

Changelog: [Internal]

Differential Revision: D108027810

Author: sammy-SC