fix(#3636): allow composite tools to use filtered backend tools

[jerm-dro]

Summary

This PR fixes issue #3636 where tools filtered via aggregation.tools[].filter could not be used in composite tool steps. The fix ensures that Filter behaves consistently with ExcludeAll - both hide tools from MCP clients while preserving them in the internal routing table for composite tool workflows.

Root Cause: The Filter configuration was applied in processBackendTools during capability discovery, which removed filtered tools from the pipeline entirely. This meant they never reached the routing table. In contrast, ExcludeAll was correctly applied later in MergeCapabilities, preserving tools in the routing table while hiding them from clients.

Fix: Move Filter logic to be applied in shouldAdvertiseTool (alongside ExcludeAll), ensuring filtered tools remain available for composite tool routing.

Changes

Core Fix

• pkg/vmcp/aggregator/tool_adapter.go: Modified processBackendTools to only apply Overrides, not Filter
• pkg/vmcp/aggregator/default_aggregator.go: Updated shouldAdvertiseTool to check Filter configuration in addition to ExcludeAll

Tests

• pkg/vmcp/aggregator/default_aggregator_test.go: Added unit tests for Filter behavior including partial matches and precedence with global ExcludeAllTools
• pkg/vmcp/aggregator/tool_adapter_test.go: Updated tests to reflect that Filter is no longer applied in processBackendTools
• test/e2e/.../virtualmcp_composite_hidden_tools_test.go: New E2E test that creates two backends (one with ExcludeAll, one with Filter) and verifies a composite tool can use hidden tools from both

Documentation

• pkg/vmcp/config/config.go: Enhanced documentation for ExcludeAllTools, ExcludeAll, and Filter fields to clarify visibility vs routing semantics
• Updated terminology from "LLM" to "MCP client" for accuracy
• Regenerated CRDs with updated field descriptions

Test Plan

• Unit tests pass (go test ./pkg/vmcp/aggregator/...)
• E2E test passes with two backends using different hiding mechanisms
• Verified composite tool successfully calls tools from both ExcludeAll and Filter backends
• Verified only the composite tool is exposed to clients (backend tools hidden)

Fixes #3636

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.74%. Comparing base (788057b) to head (1e7cb08).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3637      +/-   ##
==========================================
- Coverage   65.75%   65.74%   -0.01%     
==========================================
  Files         411      411              
  Lines       40805    40811       +6     
==========================================
  Hits        26833    26833              
- Misses      11882    11886       +4     
- Partials     2090     2092       +2     

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