Summary

Implements defaultResults for composite tool workflow steps, allowing skipped steps (due to conditions evaluating to false) or failed steps (with onError.action: continue) to provide fallback output values for downstream step templates.

Closes #2989

Large PR Justification

A significant chunk of this PR is tests and documentation. If reviewers would like, I could split the validation logic into a separate PR or I could remove it entirely, because validation is a nice-to-have.

Changes

Schema & Configuration

• Added defaultResults field to WorkflowStep CRD type (map[string]runtime.RawExtension)
• Added corresponding fields in raw config, internal config, and composer types
• Updated all converters (CRD→config, YAML→config, config→composer)

Validation

• Implemented shared validation in workflow_validation.go
• Validates that defaultResults[field] is specified when:
  • Step may be skipped (has condition or onError.action: continue)
  • Downstream steps reference that specific output field
• Validation is reused by both VirtualMCPServer and VirtualMCPCompositeToolDefinition webhooks

Template Reference Extraction

• Created pkg/templates/references.go with AST-based template parsing
• Uses text/template/parse package for reliable reference extraction
• Extracts .steps.<stepID>.output.<field> references for validation

Runtime

• Updated RecordStepSkipped to accept and store defaultResults as step output
• Updated handleToolStepFailure to set defaultResults when continuing on error

Documentation

• Added "Default Results" section to virtualmcpcompositetooldefinition-guide.md
• Added quick reference to composite-tools-quick-reference.md
• Added "Pattern 5: Default Results for Skippable Steps" to advanced-workflow-patterns.md

Example Usage

steps:
  - id: optional_enrichment
    type: tool
    tool: enrichment.api
    condition: "{{.params.enable_enrichment}}"
    arguments:
      data: "{{.params.input}}"
    defaultResults:
      text: "no enrichment performed"

  - id: process_result
    type: tool
    tool: processor.handle
    dependsOn: [optional_enrichment]
    arguments:
      # Works whether optional_enrichment ran or was skipped
      enriched_data: "{{.steps.optional_enrichment.output.text}}"

Testing

• Unit tests: Template reference extraction, validation logic
• Integration tests:
  • TestVMCPServer_DefaultResults_ConditionalSkip - verifies defaults used when condition=false
  • TestVMCPServer_DefaultResults_ContinueOnError - verifies defaults used when step fails with continue
• E2E test: virtualmcp_composite_defaultresults_test.go - operator integration with conditional step

Notes

During implementation, discovered that nested access into backend tool results (e.g., {{.steps.step1.output.message}}) doesn't work as documented - backend tool calls store text content under a "text" key. See #2994 for more details.