[C#] [Unity 6] Added logic to use either getWasmTableEntry or dynCall for C# WebGL

[rekhoff]

Description of Changes

This is a fix for #4959.

Unity 6 upgraded to a newer Emscripten version that removed the dynCall() library function. The SpacetimeDB C# SDK's WebSocket.jslib plugin used dynCall() in 6 places to invoke C# callbacks from JavaScript WebSocket events.

This PR adds a $WebSocketDynCall helper function that detects which API is available at runtime:

• On Unity 6+: uses getWasmTableEntry(ptr).apply(null, args) (direct WASM function table access)
• On Unity 2022 and earlier: uses the legacy dynCall(sig, ptr, args)

All dynCall call sites in WebSocket.jslib now route through this helper, ensuring we maintain backward-compatiblity.

API and ABI breaking changes

No breaking changes. This is a backward-compatible fix that works on both older and newer Unity versions.

Expected complexity level and risk

1. Very low risk. The change is isolated to the WebGL jslib plugin and uses runtime feature detection to choose the appropriate calling mechanism. Both code paths are well-tested:
   • Unity 2022.3.62f2: uses legacy dynCall path
   • Unity 6000.4.5f1: uses getWasmTableEntry path

Testing

• Create test server module with reducers and tables
• Unity 2022.3.62f2 Editor: Connect and subscribe works
• Unity 2022.3.62f2 WebGL Build: Connect and subscribe works
• Unity 6000.4.5f1 Editor: Connect and subscribe works
• Unity 6000.4.5f1 WebGL Build: Connect and subscribe works (was previously failing with dynCall is not defined)