flow-error-object-serialization.md

@objectstack/service-automation	patch

fix(automation): flow string templates serialize object tokens readably, never [object Object] (#3450)

A flow string field that embeds an object-valued token — most notably the engine's $error ({nodeId, message, ...}, set on a failed step) in a fault handler's notify body — rendered as the useless [object Object]. The multi-token branch of interpolateString coerced every value with String(), and notify-node did the same for a sole {$error} token.

• New shared stringifyForTemplate helper (builtin/template.ts): objects and arrays are JSON-serialized (so the text stays legible and still carries the message), primitives pass through, null/undefined render as ''.
• interpolateString's embedded-substitution branch and notify-node's title/body coercion use it. The sole-token branch still returns the raw value (typed config fields keep their type), and {$error.message} still resolves to just the message string — the documented, cleanest author form.

Split from #3425 (the readonly-strip half shipped in #3465).