test: cover Lua encode error paths

[membphis]

Summary

• Add Lua-level encode error coverage for non-finite numbers and unsupported Lua types.
• Cover non-string object keys and document the current sparse-array rejection policy.
• Assert empty object vs empty array encoding stays distinct and valid nested structures still encode.

Tests

• DYLD_LIBRARY_PATH=./target/release LUA_PATH='./lua/?.lua;/usr/local/openresty/lualib/?.lua;/usr/local/openresty/lualib/?/init.lua;;' LUA_CPATH='./vendor/lua-cjson/?.so;./target/release/lib?.so;./?.so;/usr/local/openresty/lualib/?.so;/usr/local/lib/lua/5.1/?.so;/usr/local/openresty/luajit/lib/lua/5.1/?.so' env PATH="/Users/yuanshengwang/.luarocks/bin:$PATH" busted --lua=/opt/homebrew/bin/luajit tests/lua/encode_errors_spec.lua --lpath='./lua/?.lua'
• env PATH="/Users/yuanshengwang/.luarocks/bin:$PATH" make test
• make lint
• cargo test --release --no-default-features
• cargo test --features test-panic --release

Closes #65

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 71981657-20ba-42fa-9612-ed982bc0d471

📥 Commits

Reviewing files that changed from the base of the PR and between 083791e and 7f20671.

📒 Files selected for processing (1)

• tests/lua/encode_errors_spec.lua

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/issue-65-encode-error-tests

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