Reusable publish workflow + @flowiseai/observe publish pipeline (#6341)

* feat: reusable publish workflow + @flowiseai/observe publish pipeline
* chore(observe): mark package as sideEffects: false

Addresses gemini-code-assist[bot] review feedback on PR #6341.
Enables tree-shaking of unused exports for consumer bundlers.
Audited src/ — no bare side-effect imports exist, so the claim is
accurate.