Fix TPC-C initial population to match the specification

[zinal]

Correct a-string length generation (was off-by-one), W/D ZIP codes, H_DATA range, C_DISCOUNT inclusive lower bound, S_YTD numeric scale, and STATE fields that previously relied on the length bug.

Port of ydb-platform/tpcc-postgres-cpp#4.

Changes

src/import.cpp

• Rename RandomStringBenchbase → RandomString and fix off-by-one length (TPC-C §4.3.2.2)
• W_STATE / D_STATE / C_STATE: generate 2 uppercase letters (RandomUpperAlphaString(2, 2)) instead of 3
• W_ZIP / D_ZIP / C_ZIP: RandomNumericString(4) + "11111" instead of hardcoded "123456789"
• C_DISCOUNT: lower bound 0 instead of 1 (inclusive 0.0000–0.5000)
• H_DATA: length range 12–24 instead of 10–24

src/init.cpp

• S_YTD: decimal(8, 0) instead of decimal(8, 2) per TPC-C schema