fix: mitigate ReDoS in regex validation (CVE-2025-5891)

[mohitxroxx]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	#6086
License	MIT
Doc PR	https://github.com/pm2-hive/pm2-hive.github.io/pulls

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Summary of changes:

Prevent potential Regular Expression Denial of Service (ReDoS) in Config._valid() by:

• Adding a maximum input length guard before regex execution
• Caching compiled RegExp instances
• Wrapping RegExp construction in try/catch

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