Fix contentMode() not being called as a function

[jmandel]

Summary

• Six places in expand.js, related.js, and validate.js referenced cs.contentMode (the function object) instead of cs.contentMode() (calling it)
• This caused error messages to contain the stringified function body (e.g., "is a contentMode() { return this.codeSystem.content; }") instead of the actual content mode value
• Also caused addParamUri to receive a function instead of a string, and content mode comparisons to silently fail since function !== 'complete' is always true

Test plan

• Expand a ValueSet referencing a fragment CodeSystem (e.g., http://hl7.org/fhir/sid/icd-9-cm) — error message should say "is a fragment" not "is a contentMode() {...}"
• Expand with incomplete-ok=true for a fragment CodeSystem — addParamUri should receive the string value, not a function
• $validate-code against a fragment/example CodeSystem — contentMode.value should be a string like "fragment", not a function reference

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