fix: upgrade raft-log: use pread to prevent race condition in concurrent chunk reads (databendlabs#19401)

Chunk::read_record() had a race condition when multiple threads read
from the same chunk concurrently. The non-atomic seek + read operation
allowed one thread's seek to be overwritten by another before reading,
causing reads from wrong file positions.

Changes:
- Use read_exact_at() (pread) instead of seek() + read() in read_record()
- Add concurrent read test to verify the fix

Author: drmingdrmer

Cargo.lock

@@ -395,7 +395,7 @@ proptest = { version = "1", default-features = false, features = ["std"] }
 prost = { version = "0.13" }
 prost-build = { version = "0.13" }
 prqlc = "0.11.3"
-raft-log = { version = "0.2.12" }
+raft-log = { version = "0.2.13" }
 rand = { version = "0.8.5", features = ["small_rng", "serde1"] }
 rand_distr = "0.4.3"
 rayon = "1.9.0"