Remove custom string matching helpers in favor of strings.Contains

[Copilot]

The test file contained hand-rolled containsString and searchString functions that duplicated strings.Contains from the standard library.

Changes

• init_from_code_test.go: Removed containsString and searchString; updated containsAll to call strings.Contains directly; added "strings" import.

// Before
func containsAll(s string, substrings ...string) bool {
    for _, sub := range substrings {
        if !containsString(s, sub) { ... }
    }
}
func containsString(s, substr string) bool { return len(s) >= len(substr) && searchString(s, substr) }
func searchString(s, substr string) bool   { /* manual byte-by-byte scan */ }

// After
func containsAll(s string, substrings ...string) bool {
    for _, sub := range substrings {
        if !strings.Contains(s, sub) { ... }
    }
}

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