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.

[MyTH-zyxeon]

Review-assist note for the Bitgesell #81 improvement lane.

I checked the public PR body and diff. This looks like a small API validation/testability fix:

• preserves transaction pointer-list validation failures instead of collapsing them into a generic JSON-decode path
• corrects the batch transaction-hash pointer limit message to the actual 500-item limit
• adds focused unit coverage for list limits, non-list JSON, and mixed pointer/hash parsing
• stays inside request validation and unit-test code; no live node/RPC credentials, wallet, signing, or chain-action path changes

Suggested maintainer checks before merge:

• run python3 -m pytest test/unit/test_transaction_pointer_list.py -q or the repo's normal unit suite
• confirm malformed pointer-list responses match the API's existing error-code contract
• confirm happy-path transaction pointer and hash lookups keep the same response shape

From the public patch shape, this looks like a low-risk, reviewable API correctness fix for the Bitgesell bounty/improvement queue.