Bulk Parallel Snapshot Sources

spec: /specs/40_Snapshots/spec.md#bulk-parallel-snapshots

Execute Fast Parallel (parallel-chunk) snapshot on multiple sources.

Mirrors the per-source POST /sources/execute_parallel_snapshot semantics fanned out across all targeted sources: seeds user intent into Mongo.sources.snapshotting_tables; the in-process orchestrator picks up on its next cycle. No KC signal-table writes, no row/table locks.

Supports two modes for source selection:

1. Explicit IDs: Pass a list of source IDs
2. Select all: Set select_all=true with optional filters

Returns partial-success results — continues even when individual sources fail (e.g. regex source with no table.include.list).