feat(pipeline): implement FHIR validation pipeline step

Add a new validation step that validates FHIR resources against an
external FHIR validation service via HTTP POST to /validateResource.

The step produces per-file OperationOutcome reports in the validation/
directory and fails if any resource has error-level or fatal-level
issues. It supports configurable concurrent validation requests and
resumption by skipping files that already have reports.

Validation is a "transparent" step — it doesn't produce FHIR data
output, so subsequent pipeline steps look past it to find their input
from the previous data-producing step.

Closes #7

Author: trobanga

cmd/pipeline.go

@@ -223,7 +223,20 @@ func executeStep(job *models.PipelineJob, stepName models.StepName, config *mode
 		return nil
 
 	case models.StepValidation:
-		fmt.Println("Validation step not yet implemented - job will remain at this step")
+		fmt.Println("Starting validation step...")
+		if err := pipeline.ExecuteValidationStep(job, jobDir, logger); err != nil {
+			failedJob := pipeline.FailJob(job, err.Error())
+			if saveErr := pipeline.UpdateJob(config.JobsDir, failedJob); saveErr != nil {
+				logger.Error("Failed to save job state", "error", saveErr)
+			}
+			return fmt.Errorf("validation step failed: %w", err)
+		}
+
+		if err := pipeline.UpdateJob(config.JobsDir, job); err != nil {
+			return fmt.Errorf("failed to save job state: %w", err)
+		}
+
+		fmt.Printf("\n✓ Validation completed\n")
 		return nil
 
 	case models.StepCSVConversion:

internal/models/config.go

@@ -40,6 +40,7 @@ func DefaultCompressionConfig() CompressionConfig {
 // ServiceConfig contains connection details for external HTTP services
 type ServiceConfig struct {
 	DIMP              DIMPConfig              `yaml:"dimp" json:"dimp"`
+	Validation        ValidationConfig        `yaml:"validation" json:"validation" mapstructure:"validation"`
 	CSVConversion     CSVConversionConfig     `yaml:"csv_conversion" json:"csv_conversion"`
 	ParquetConversion ParquetConversionConfig `yaml:"parquet_conversion" json:"parquet_conversion"`
 	TORCH             TORCHConfig             `yaml:"torch" json:"torch"`
@@ -48,6 +49,12 @@ type ServiceConfig struct {
 	LocalImport       LocalImportConfig       `yaml:"local_import" json:"local_import" mapstructure:"local_import"`
 }
 
+// ValidationConfig contains FHIR validation service settings
+type ValidationConfig struct {
+	URL                   string `yaml:"url" json:"url" mapstructure:"url"`
+	MaxConcurrentRequests int    `yaml:"max_concurrent_requests" json:"max_concurrent_requests" mapstructure:"max_concurrent_requests"`
+}
+
 // LocalImportConfig contains settings for local directory import
 type LocalImportConfig struct {
 	Dir string `yaml:"dir" json:"dir" mapstructure:"dir"` // Default directory path for local imports
@@ -371,6 +378,8 @@ func (c *ServiceConfig) HasServiceURL(step StepName) bool {
 	switch step {
 	case StepDIMP:
 		return c.DIMP.URL != ""
+	case StepValidation:
+		return c.Validation.URL != ""
 	case StepCSVConversion:
 		return c.CSVConversion.URL != ""
 	case StepParquetConversion:
@@ -380,7 +389,7 @@ func (c *ServiceConfig) HasServiceURL(step StepName) bool {
 	case StepSend:
 		return c.Send.IsConfigured()
 	default:
-		return true // Import and validation don't require external services
+		return true // Import steps don't require external services
 	}
 }
 
@@ -389,6 +398,8 @@ func (c *ServiceConfig) GetServiceURL(step StepName) string {
 	switch step {
 	case StepDIMP:
 		return c.DIMP.URL
+	case StepValidation:
+		return c.Validation.URL
 	case StepCSVConversion:
 		return c.CSVConversion.URL
 	case StepParquetConversion:

internal/models/validation.go

@@ -162,7 +162,7 @@ func (c *ProjectConfig) Validate() error {
 	for _, step := range c.Pipeline.EnabledSteps {
 		if !c.Services.HasServiceURL(step) {
 			switch step {
-			case StepDIMP, StepCSVConversion, StepParquetConversion, StepSend:
+			case StepDIMP, StepValidation, StepCSVConversion, StepParquetConversion, StepSend:
 				return fmt.Errorf("service URL required for enabled step '%s'", step)
 			}
 		}
@@ -190,6 +190,11 @@ func (c *ProjectConfig) Validate() error {
 			return fmt.Errorf("invalid dimp url: %w", err)
 		}
 	}
+	if c.Services.Validation.URL != "" {
+		if _, err := url.Parse(c.Services.Validation.URL); err != nil {
+			return fmt.Errorf("invalid validation url: %w", err)
+		}
+	}
 	if c.Services.CSVConversion.URL != "" {
 		if _, err := url.Parse(c.Services.CSVConversion.URL); err != nil {
 			return fmt.Errorf("invalid csv_conversion url: %w", err)
@@ -296,6 +301,9 @@ func (c *ProjectConfig) ValidateServiceConnectivity(transport *http.Transport) e
 		case StepDIMP:
 			serviceURL = c.Services.DIMP.URL
 			serviceName = "DIMP"
+		case StepValidation:
+			serviceURL = c.Services.Validation.URL
+			serviceName = "Validation"
 		case StepSend:
 			serviceURL = c.Services.GetServiceURL(StepSend)
 			serviceName = "Send"