Use valid FSE dictionary costs in optimal parser

[Rohithmatham12]

Fixes #4681.

ZSTD_loadCEntropy() already marks dictionary FSE tables that contain zero-probability symbols as FSE_repeat_check, because they are not safe to assume valid for the full symbol range. ZSTD_rescaleFreqs() ignored those per-stream repeat modes and entered the dictionary-derived FSE pricing path whenever the Huffman table was HUF_repeat_valid.

For tiny dictionaries with optimal-parser strategies, that can make FSE_getMaxNbBits() return its documented fake cost for zero-frequency symbols (tableLog + 1). With scaleLog = 10, this reaches the assert(bitCost < scaleLog) in the optimal parser.

This changes first-block optimal-parser pricing to decide per entropy stream:

• use dictionary HUF costs only when HUF repeat mode is valid,
• use dictionary FSE costs only when that specific FSE repeat mode is valid,
• otherwise fall back to the existing baseline stats for that stream.

That preserves dictionary-derived pricing when the table is known complete, while avoiding invalid costs from partial FSE tables.

Test plan:

• make -j4 fuzzer
• ./fuzzer -i1 -s1 -v
• git diff --check