fix(bigquery-firestore-export): DISPLAY_NAME edits silently no-op on update

[cabljac]

DISPLAY_NAME is immutable: true in the extension but mutable in the kit, and constructUpdateTransferConfigRequest never adds display_name to the DTS update mask. Editing it and redeploying reports success and changes nothing.

Fix in #2980: display_name joins the mask when it differs, alongside the existing schedule and notification_pubsub_topic comparisons. Verified against a live transfer config. The alternative, making the param immutable to match the extension, was rejected in favour of keeping the kit's mutable param, consistent with the forward ruling in #3030.

Parity ledger: #2974, bigquery-firestore-export DISPLAY_NAME params row.