Update the schema to remove stdio transport type for remote servers (#343)

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## Motivation and Context
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Addresses
https://github.com/modelcontextprotocol/registry/pull/340#discussion_r2316217614
## How Has This Been Tested?
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tested? -->

## Breaking Changes
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No
## Types of changes
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- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
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- [ ] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [ ] My code follows the repository's style guidelines
- [ ] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

## Additional context
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Signed-off-by: Radoslav Dimitrov <radoslav@stacklok.com>

docs/server-json/server.schema.json

@@ -334,8 +334,7 @@
           "type": "string",
           "enum": [
             "streamable-http",
-            "sse",
-            "stdio"
+            "sse"
           ],
           "description": "Transport protocol type",
           "example": "sse"