fix(embed): preserve legacy grpc-gateway error field via internal proto

[fuweid]

grpc-gateway v2.0 dropped the legacy "error" JSON field, breaking clients relying on it (grpc-ecosystem/grpc-gateway#1098, grpc-ecosystem/grpc-gateway#1242). Add internalpb.GRPCGatewayError and rewrite Status responses to preserve backward-compatible error payloads.

Deprecation plan: keep "error" alongside "message" in v3.7 with documentation warning; remove "error" in v3.8 once clients migrate.

Fixes: #21100

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.38%. Comparing base (c492848) to head (820349e).

Files with missing lines	Patch %	Lines
server/embed/serve.go	0.00%	9 Missing and 1 partial ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
server/embed/serve.go	57.28% <0.00%> (-2.11%)	⬇️

... and 18 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #21143      +/-   ##
==========================================
- Coverage   68.40%   68.38%   -0.03%     
==========================================
  Files         429      429              
  Lines       35242    35251       +9     
==========================================
- Hits        24109    24107       -2     
- Misses       9737     9752      +15     
+ Partials     1396     1392       -4     

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