snapshot: set `ReplicaRead` to false when `ReplicaReadType` fallbacks to `ReplicaReadLeader` (#1663)

[you06]

Cherry pick #1663 to tidb-8.5

---

ref pingcap/tidb#61745

The replicaReadAdjuster can choose leader and follower replica after a request is created. When the request is created as a replica-read request but fallback to leader read by replicaReadAdjuster, we need to set the replica_read to false as well. Unless it can perform a meaningless read-index in leader and harm the performance.

Before this PR

Many read-index proposals.

This PR

Few read-index proposals.

[you06]

@glorv @zyguan I pushed an empty commit to trigger CI and the LGTM tag is removed, so I need your approve again, thanks.

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: glorv, zyguan

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[LGTM Timeline notifier]

Timeline:

• 2025-06-30 07:41:10.482232131 +0000 UTC m=+1294323.205411112: :ballot_box_with_check: agreed by glorv.
• 2025-06-30 08:26:15.488228585 +0000 UTC m=+1297028.211407566: :ballot_box_with_check: agreed by zyguan.
• 2025-07-01 08:14:49.995013009 +0000 UTC m=+1382742.718191985: :heavy_multiplication_x::repeat: reset by you06.
• 2025-07-01 08:54:31.48545135 +0000 UTC m=+1385124.208630327: :ballot_box_with_check: agreed by glorv.
• 2025-07-01 11:08:42.496982418 +0000 UTC m=+1393175.220161397: :ballot_box_with_check: agreed by zyguan.