Add SDC Agents community toolsets module

[twcook]

Summary

• Adds google.adk_community.sdc_agents module: thin re-export wrapper over the sdc-agents PyPI package
• Adds sdc-agents as an optional dependency (pip install google-adk-community[sdc-agents])
• Includes import verification tests and a sample agent demo

Details

SDC Agents provides eight BaseToolset implementations (32 tools) for semantic data governance:

Toolset	Purpose
CatalogToolset	Discover published SDC4 schemas and download artifacts
IntrospectToolset	Analyze datasource structure (SQL, CSV, JSON, MongoDB)
MappingToolset	Match columns to schema components by type and name
AssemblyToolset	Compose data models from catalog components
GeneratorToolset	Generate validated XML instances
ValidationToolset	Validate and digitally sign XML instances
DistributionToolset	Deliver RDF triples to graph databases
KnowledgeToolset	Index domain documentation for semantic search

The module is a thin re-export wrapper, keeping sdc-agents on PyPI as the single source of truth. The optional dependency ensures no impact on users who don't need SDC toolsets.

A companion PR for the adk-docs repo adds a community resource card.

Test plan

• pip install -e ".[sdc-agents]" installs sdc-agents successfully
• pytest tests/unittests/test_sdc_agents_imports.py passes (all re-exports resolve)
• Without sdc-agents installed, tests skip gracefully and google.adk_community still imports
• Sample in contributing/samples/sdc_agents_demo/ runs with adk run .

CLA: Signed

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[twcook]

@googlebot I signed it.