fix(RpcClient): throw on non-success HTTP before parsing JSON

[cschuchardt88]

Send/SendAsync parsed the body even when the HTTP status was 4xx/5xx. Proxy HTML and gateway errors became JSON parse failures instead of HttpRequestException.

JSON-RPC errors still arrive as HTTP 200 with an error object and are unchanged.

Independent of other PRs. RpcClient only; not consensus-critical.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 51.42%. Comparing base (2e491a2) to head (afbaa96).

Files with missing lines	Patch %	Lines
plugins/RpcClient/RpcClient.cs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           master-n3    #1101      +/-   ##
=============================================
- Coverage      51.42%   51.42%   -0.01%     
=============================================
  Files            284      284              
  Lines          16677    16679       +2     
  Branches        2143     2143              
=============================================
+ Hits            8576     8577       +1     
- Misses          7527     7528       +1     
  Partials         574      574              

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