Migrate all usages of the deprecated math/rand package to math/rand/v2
across the codebase. This fixes the Go 1.25 compatibility issue where
rand.Seed no longer works (ClickHouse#1652).

Key changes:
- Replace math/rand imports with math/rand/v2 in all 19 affected files
- Use rand.IntN instead of rand.Intn (v2 naming convention)
- Replace rand.NewSource with rand.NewPCG for local random generators
- Remove all rand.Seed calls (v2 auto-seeds the global source)
- Fix ConnOpenRandom strategy: pick random offset once outside the
  loop instead of on every iteration, ensuring all addresses are
  tried before failing over
- Re-enable TestConnFailoverRandom and TestStdConnFailoverRandom
  tests that were skipped due to Go 1.25 math/random changes

Fixes ClickHouse#1653