[task] Consolidate scattered validation functions into dedicated validation files #3536

Objective

Move scattered validation functions from various files into dedicated *_validation.go files to improve code organization and discoverability.

Context

Analysis identified 20+ validation functions scattered across multiple files outside of dedicated validation files. This reduces discoverability and makes it harder to maintain consistent validation patterns.

Part of issue #3435.

Approach

1. Create new validation files

• expression_validation.go - For expression safety validation functions
• engine_validation.go - For engine validation functions
• template_validation.go - For template validation functions
• mcp_config_validation.go - For MCP config validation functions

2. Move validation functions

From expression_safety.go:

• validateExpressionSafety(expr string) error
• validateSingleExpression(expr string, location string) error

From engine.go:

• (*Compiler).validateEngine(engine string) error
• validateSingleEngineSpecification(engines []string) error

From template.go:

• validateNoIncludesInTemplateRegions(...) error

From mcp-config.go:

• ValidateMCPConfigs(...) error
• validateStringProperty(...) error
• validateMCPRequirements(...) error

From strict_mode.go:

• Move validation logic to existing validation_strict_mode.go

3. Update imports

• Update all files that call these validation functions
• Ensure no circular dependencies are introduced
• Update test files as needed

Files to Modify

Create:

• pkg/workflow/expression_validation.go
• pkg/workflow/engine_validation.go
• pkg/workflow/template_validation.go
• pkg/workflow/mcp_config_validation.go

Update:

• pkg/workflow/expression_safety.go (remove moved functions)
• pkg/workflow/engine.go (remove moved functions)
• pkg/workflow/template.go (remove moved functions)
• pkg/workflow/mcp-config.go (remove moved functions)
• pkg/workflow/strict_mode.go (remove moved functions)
• Any files that import these validation functions

Acceptance Criteria

• All validation functions are in dedicated *_validation.go files
• No validation functions remain in non-validation files (except inline validation)
• All imports are updated correctly
• All existing tests pass (make test-unit)
• No circular dependencies introduced
• Code follows existing validation file patterns
  Related to [refactor] 🔧 Semantic Function Clustering Analysis - Code Organization Opportunities #3435

AI generated by Plan Command for #3435