test(router): update MCP input-schema goldens to union nullable form

[asoorm]

Updates two stale goldens in router-tests/protocol/mcp_test.go to unblock the ./protocol job on #2925.

The engine bump (graphql-go-tools #1518/#1528) changed nullable rendering from {"type":"x","nullable":true} to the union {"type":["x","null"]} (nullable enums gain a trailing null). These goldens hard-code the generated schema, so they went stale. Engine fix is correct; goldens just needed updating.

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[github-actions]

Router image scan passed

✅ No security vulnerabilities found in image:

ghcr.io/wundergraph/cosmo/router:sha-32381a5d1c4e6bea8291d3de32816c57dd9d58a4

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.27%. Comparing base (a58f2c8) to head (a113811).

Additional details and impacted files

@@                                            Coverage Diff                                             @@
##           yury/eng-9581-inline-fragments-on-abstract-list-types-charge-all-items    #2926      +/-   ##
==========================================================================================================
+ Coverage                                                                   63.18%   66.27%   +3.09%     
==========================================================================================================
  Files                                                                         258      258              
  Lines                                                                       27309    27309              
==========================================================================================================
+ Hits                                                                        17254    18100     +846     
+ Misses                                                                       8653     7773     -880     
- Partials                                                                     1402     1436      +34     

see 26 files with indirect coverage changes

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