fix: preserve transaction pointer list validation errors

[JasonZhouPW]

Summary

• Preserves parameter validation errors for transaction pointer list endpoints instead of converting them to JSON decode errors.
• Fixes the batch transaction hash pointer limit message to report the actual 500-item limit.
• Adds focused unit coverage for list limits, non-list JSON, and mixed pointer/hash parsing.

Validation

• python3 -m pytest test/unit/test_transaction_pointer_list.py -q
• PYTHONDONTWRITEBYTECODE=1 python3 AST syntax check
• git diff --check

Bounty context: Bitgesell improvement / PR bounty program, related to BitgesellOfficial/bitgesell#81 and BitgesellOfficial/bitgesell#32. Payout details can be provided after maintainer approval.