refactor(test): replace custom string helpers with strings.Contains in resume_test.go

[lizhengfeng101]

Description

internal/session/resume_test.go defines two custom helper functions (contains and containsSubstring) that reimplement strings.Contains from the standard library:

func contains(s, substr string) bool {
	return len(s) >= len(substr) && (s == substr || len(s) > 0 && containsSubstring(s, substr))
}

func containsSubstring(s, sub string) bool {
	for i := 0; i <= len(s)-len(sub); i++ {
		if s[i:i+len(sub)] == sub {
			return true
		}
	}
	return false
}

These should be replaced with a single call to strings.Contains, which is functionally identical and far more readable.

Scope

• File: internal/session/resume_test.go
• Functions to remove: contains, containsSubstring
• Call site: TestSessionFilePath_ValidID

Acceptance Criteria

• Remove the contains and containsSubstring helper functions
• Add "strings" to the import block
• Replace the call site with strings.Contains(path, expectedSuffix)
• Tests pass (make test)
• Code check passes (make check)

Context

Discovered during code review of #590. The custom helpers are unnecessary since strings.Contains provides the same functionality. This is a straightforward cleanup.