feat: proxyd update consensus mode out of range requests to be api-spec compliant

[jelias2]

Description

• In consensus mode proxyd if requests were out of range for the consensus backend they would return an http 400 status code and error message. This PR updates proxyd to be json-rpc spec complaint by returning a compatible response to the client. This should ensure the client can properly function regardless of proxyd intercepting requests or not

Tests

Please describe any tests you've added. If you've added no tests, or left important behavior untested, please explain why not.

Additional context

Add any other context about the problem you're solving.

Metadata

• Fixes #[Link to Issue]

[codecov-commenter]

Codecov Report

❌ Patch coverage is 47.61905% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.59%. Comparing base (f9bf450) to head (a8a0851).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
proxyd/backend.go	47.61%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #552      +/-   ##
==========================================
- Coverage   57.61%   57.59%   -0.02%     
==========================================
  Files          94       94              
  Lines       14204    14224      +20     
==========================================
+ Hits         8184     8193       +9     
- Misses       5522     5534      +12     
+ Partials      498      497       -1     

Flag	Coverage Δ
proxyd	71.22% <47.61%> (-0.11%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
proxyd/backend.go	79.26% <47.61%> (-0.57%)	⬇️

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[geoknee]

replaces #547