test: share gRPC plugin header forwarding tests

[asoorm]

Optional follow-up on #2877. Feel free to take or leave.

Dedupes the standard and OCI header-forwarding suites via a shared testenv.RunGRPCPluginHeaderCases helper.

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[github-actions]

Router image scan passed

✅ No security vulnerabilities found in image:

ghcr.io/wundergraph/cosmo/router:sha-3c64c9c3900cd73a2feed1927297cb341e336c7b

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.28%. Comparing base (a5765a1) to head (e98500d).

Additional details and impacted files

@@                                               Coverage Diff                                                @@
##           ludwig/eng-9615-router-plugin-metadata-overwritten-in-grpc-plugin-middleware    #2883      +/-   ##
================================================================================================================
+ Coverage                                                                         66.24%   66.28%   +0.03%     
================================================================================================================
  Files                                                                               258      258              
  Lines                                                                             27064    27064              
================================================================================================================
+ Hits                                                                              17929    17939      +10     
+ Misses                                                                             7723     7713      -10     
  Partials                                                                           1412     1412              

see 4 files with indirect coverage changes

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