Fix __index dispatch for string.gsub replacement tables

[XinyuGuan]

Fixes #359.

string.gsub currently uses the raw C# indexer for replacement tables, so a missing key never reaches __index. Use the existing GetTableAsync lookup with the cancellation token. Raw hits keep their precedence; function handlers, table chains and asynchronous handlers follow the existing language-level lookup path.

Adds 11 regression cases: 9 fail on the original implementation and all pass with this change. They cover capture keys, replacement limits, nil/false/numeric results, protected errors and state reuse as well as synchronous/asynchronous handlers. The reproducer's fixed output matches PUC-Rio Lua 5.2.3 byte-for-byte in two repetitions.

Validation on Linux x64, SDK 10.0.401 / runtime 8.0.32:

• DOTNET_GCHeapHardLimit=0x40000000 dotnet test tests/Lua.Tests/Lua.Tests.csproj -c Release --filter "TestCategory!=ExpectedFailure": 567 passed (unchanged baseline: 556 passed).
• dotnet build src/Lua/Lua.csproj -c Release: all four target frameworks build.
• dotnet csharpier check . and git diff --check: pass.

The heap limit is needed on this server for the existing large-allocation tests; without it the unmodified suite was killed by the OS. The project-defined ExpectedFailure filter is unchanged. No host-indexer behavior changes are included.

[akeit0]

Thanks!