resourcecontrol: bypass RU accounting for NextGen background activities

[YuhaoZhang00]

Goal

Under the nextgen build tag, expand shouldBypass() so selected heavy internal operations do not consume the user-facing RU quota.

Scope

• DDL index backfill: add_index and merge_temp_index
• BR: br
• IMPORT INTO: ImportInto
• Workload Learning: WorkloadLearning

Non-goals

This issue does not track request-source RU metrics or RequestSource() plumbing. That observability work is tracked separately by #1929 and tikv/pd#10588.

Status

Parked until the product requirement is reactivated.

Tracking issue (TiDB side): pingcap/tidb#64339

Implementation PR: #1930