Composite Tools Support Structured Content

[jerm-dro]

Stacked on #3006 because of when I discovered this gap.

Summary

• Fix issue where composite tool steps could not access nested fields from backend tool responses
• Backend HTTP client now checks for StructuredContent before falling back to Content array processing (legacy behavior)
• Add integration tests for structured content access and nested defaultResults
• Document step output format and numeric comparison behavior
• I slightly deviated from the solution proposed in 2994, because I felt namespacing all data behind structuredContent would be verbose/clunky. It also does not match much of the existing documentation.

Fixes #2994

Changes

Core Fix (pkg/vmcp/client/client.go):

• Check result.StructuredContent before processing result.Content array
• If StructuredContent is an object (map[string]any), use it directly as step output
• Fall back to existing Content processing for text/image content

Test Helpers (test/integration/vmcp/helpers/backend.go):

• Add StructuredHandler field to BackendTool for structured content responses
• Add NewBackendToolWithStructuredResponse() constructor
• Update CreateBackendServer to use mcp.NewToolResultStructuredOnly() when appropriate

Integration Tests (test/integration/vmcp/vmcp_integration_test.go):

• TestVMCPServer_StructuredContent: Tests nested field access via {{.steps.X.output.field.nested}}
• TestVMCPServer_DefaultResults_NestedStructure: Tests nested defaultResults fallback

Documentation:

• docs/operator/virtualmcpcompositetooldefinition-guide.md: Added "Step Output Format" and "Numeric Values in Templates" sections
• docs/operator/advanced-workflow-patterns.md: Added "Step Output Format" and "Numeric Comparisons" sections

[codecov]

Codecov Report

❌ Patch coverage is 0% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.40%. Comparing base (a748373) to head (2fcadb4).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
test/integration/vmcp/helpers/backend.go	0.00%	12 Missing ⚠️
pkg/vmcp/client/client.go	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3009      +/-   ##
==========================================
- Coverage   56.43%   56.40%   -0.03%     
==========================================
  Files         333      333              
  Lines       33001    33019      +18     
==========================================
+ Hits        18623    18626       +3     
- Misses      12796    12811      +15     
  Partials     1582     1582              

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[jerm-dro]

Tests seems stubbornly flaky on this branch. I'm going to debug on this stacked branch:

#3028

I'll wait to merge this branch until I'm convinced it's not increasing the flakiness.