fix(lighthouse): avoid mutating cached preset config

[Kikobeats]

Summary

• Shallow-copy the preset config object before merging user settings, so the cached import() module export is never mutated across calls
• Prevents settings from one getConfig() call leaking into subsequent calls using the same preset

Context

import() caches module exports. The previous code assigned config.settings = { ... } directly on the cached object, so user-provided settings (e.g. skipAudits, output) from one request would persist into the next request using the same preset within the same process.

Closes #697

Made with Cursor

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Note

Low Risk
Low risk: small, localized change to config construction that avoids mutating imported preset objects; behavior should be identical aside from preventing cross-call settings leakage.

Overview
Fixes get-config to avoid mutating the imported Lighthouse preset config (which can be cached by import()). The function now shallow-copies the preset object and merges user-provided settings into a new config.settings, preventing options from one call leaking into later calls using the same preset.

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[coveralls]

coverage: 74.723% (+0.008%) from 74.715% — fix/lighthouse-config-mutation into master