Add change_stream_v2_fcv_upgrade_downgrade.js to validate change stream v2 behavior across FCV transitions for the precise shard targeter (featureFlagChangeStreamPreciseShardTargeting).

Test cases:

FCV downgrade (v2 -> v1 fallback):

• Collection, database, cluster scope: v2 stream falls back to v1 via RetryChangeStream, cursor ID changes, targeting widens to all shards + config server
• DbAbsent state: stream survives downgrade
• Resume token: v2 resume token remains valid after downgrade
• Write interleaving: no events lost when writes interleave with downgrade

FCV upgrade (v1 -> v2):

• Existing v1 stream stays v1 after upgrade (no automatic re-evaluation)
• Pre-upgrade resume token: v2 stream with pre-upgrade startAtOperationTime initially targets all shards, then narrows after processing NamespacePlacementChanged
• Resume token: v1 resume token remains valid after upgrade