fix(mcp): resolve npx stdio connection failures

[teknium1]

Summary

• salvage the MCP stdio startup fix from PR fix: resolve MCP npx stdio connection failures #977 onto current main
• resolve bare stdio commands against the actual filtered subprocess PATH
• add targeted fallback resolution for npx / npm / node in Hermes-managed install locations
• prepend the resolved command directory back into subprocess PATH so #!/usr/bin/env node chains resolve reliably
• improve MCP discovery warnings by surfacing the actionable missing executable instead of raw ExceptionGroup noise
• add deterministic regression tests for the issue-948 npx / node startup path cases

Why salvage instead of merging the old branch directly

• PR fix: resolve MCP npx stdio connection failures #977 is stale against current main
• the original bugfix is valid, but one of the new tests was environment-dependent when npx already existed on the host PATH
• this salvage keeps the useful fix while making the regression coverage deterministic on current main

Test plan

• source /home/teknium/.hermes/hermes-agent/.venv/bin/activate && python -m pytest -o addopts='' tests/tools/test_mcp_tool.py tests/tools/test_mcp_tool_issue_948.py tests/test_model_tools.py -n0 -q

Credit

• Salvaged from PR fix: resolve MCP npx stdio connection failures #977 by @kshitijk4poor