PROD-2309: guard against unresolved content references before push

[5PK]

Summary

• Root cause: linked/nested content references with no source→target mapping were shipped with the stale source contentID; the server's batch engine dereferences it as null and throws a NullReferenceException, surfaced to operators as an opaque "Object reference not set to an instance of an object."
• Adds collectUnresolvedContentReferences and wires it into ContentBatchProcessor.prepareContentPayloads (new STEP 3.5) — skips an item before the API call with a precise, actionable reason (field path + unmapped source contentID) instead of shipping a payload guaranteed to fail server-side.
• Implements the previously-stubbed prettyException and adds formatBatchItemError in batch-polling.ts so server error messages keep the exception type and failing server method (e.g. NullReferenceException: ... [server: BatchInsertContentitem]) instead of being regex-stripped down to the bare message.
• Renames the skip log line from "Orphaned content item" to "Skipping content item N (ref)" to reflect the broader set of skip reasons.

Test plan

• npx tsc --noEmit — clean
• npx jest --roots src -- src/lib/pushers — 29 suites / 477 tests passing
• Verified against the real compiled dist modules with the actual {referencename, sortids, fulllist:false} payload shape (12 mapped→112, 13 unmapped): field mapper ships source id 13 unchanged; guard detects and skips it with the unresolved-reference message.
• Repro-verify at runtime against the reporting cluster (loyaltyconfig / eurojackpothomelinks) if a fresh repro window is available.

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