feat(oauth): add Runlayer mode to Go OAuth test server

[Dumbris]

Summary

• Add -runlayer-mode flag to Go OAuth test server that mimics Runlayer's strict validation
• Returns Pydantic-style 422 errors when RFC 8707 resource parameter is missing
• Helps test and debug issue OAuth regression for runlayer backed servers #271 compatibility with Runlayer servers

Changes

OAuth Test Server:

• Add PydanticValidationError and PydanticErrorDetail types in types.go
• Add RunlayerMode option in options.go
• Add pydanticValidationError() method in authorize.go
• Add -runlayer-mode CLI flag (implies -require-resource)

Testing:

• Add --runlayer-mode flag to scripts/run-oauth-e2e.sh
• Add test/integration/test_runlayer_mode.sh integration test

Documentation:

• Update docs/oauthserver_testing.md with Runlayer mode usage
• Add comparison table for Go vs Python OAuth test servers

Usage

# Start server in Runlayer mode
go run ./tests/oauthserver/cmd/server -port 9000 -runlayer-mode

# Test without resource - get 422 Pydantic error
curl "http://localhost:9000/authorize?client_id=test-client&..."
# Returns: {"detail":[{"type":"missing","loc":["query","resource"],"msg":"Field required"}]}

Test plan

• go test ./tests/oauthserver/... - All existing tests pass
• go vet ./tests/oauthserver/... - No issues
• ./test/integration/test_runlayer_mode.sh - New integration test passes
• Manual testing with mcpproxy against Runlayer-style mock

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