fix(pkg/paralleltestctx): detect helper-created timeout contexts

[ethanndickson]

Summary

• Detect timeout-context creation anywhere in assignment RHS expressions, not only the outermost call.
• Summarize same-package test helpers that return context.Context from known timeout constructors, including simple helper chains, so calls like ctx := chatdTestContext(t) are linted.
• Keep detection conservative by requiring context-typed assignments and scanning only test-file helpers in the analyzed package.
• Add analyzer testdata for nested wrappers, helper chains, and non-context RHS false-positive coverage.