MCP Server: Add CpgDfgBackwardTool

[lshala]

Extracted from #2574 to make the PR easier to review.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.19%. Comparing base (511ab07) to head (329c4e0).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...isec/cpg/mcp/mcpserver/tools/CpgDfgBackwardTool.kt	72.72%	2 Missing and 1 partial ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
...de/fraunhofer/aisec/cpg/mcp/mcpserver/McpServer.kt	100.00% <100.00%> (ø)
...isec/cpg/mcp/mcpserver/tools/CpgDfgBackwardTool.kt	72.72% <72.72%> (ø)

... and 3 files with indirect coverage changes

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