Convert network tools to structured output

[subpop]

Straightforward port of the network.py tools to return structured output instead of string-formatted text.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
unittests	97.38% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
src/linux_mcp_server/formatters.py	94.02% <ø> (-1.30%)	⬇️
src/linux_mcp_server/parsers.py	84.52% <100.00%> (+0.63%)	⬆️
src/linux_mcp_server/tools/network.py	100.00% <100.00%> (ø)
tests/parsers/test_merge_network_interfaces.py	100.00% <100.00%> (ø)
tests/test_formatters.py	100.00% <ø> (ø)
tests/tools/test_network.py	100.00% <100.00%> (ø)

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