[gateway] Reject duplicate MAP keys in entry arrays

[Gezi-lzq]

Purpose

Linked issue: close #4204

Canonical MAP entry arrays currently accept duplicate decoded keys. When such a row is read, a later value can silently overwrite an earlier value. This change rejects the invalid MAP at the Gateway REST boundary.

Brief change log

• Detect duplicate keys after canonical MAP entries have been decoded to their declared key type.
• Report the second occurrence with its precise v[index].key path as an invalid value.
• Add regression coverage for non-adjacent string duplicates and semantically equal integer keys (7 and "7").

Tests

• cargo test --all-targets
• cargo fmt --all -- --check
• cargo clippy --all-targets -- -D warnings -A clippy::needless_borrows_for_generic_args -A clippy::uninlined_format_args
• git diff --check

API and Format

No public API or storage format changes. Invalid canonical MAP entry arrays with duplicate keys now return HTTP 400 instead of being written.

Documentation

No documentation changes. This fixes validation of an existing API contract.

[naivedogger]

+1. Thanks for catching this. Comparing keys after schema-aware decoding is the right approach, since it also catches equivalent JSON representations such as 7 and "7". The implementation and tests look good to me. cc @beryllw

[beryllw]

LGTM! Thanks for the fix.