Rebased onto current main to drop the empty-tool_calls fix (already on
main via #86654, cherry-picked from #77944 with @webtecnica's
authorship). This PR now carries only the two fixes unique to it:
1. A pre-existing api_content sidecar left stale on the consecutive-
assistant merge. The sidecar takes priority over content at
API-build time, so a merge could silently discard its own freshly
concatenated content on the next call. Only dropped when the merge
actually changes the resulting value (wz-heng, #78063 review) --
content_rewritten compares before/after value, not just whether an
assignment branch fired, so a falsy new_content (e.g. "") that
strips to nothing no longer trips a spurious sidecar drop.
2. sanitize_api_messages never flagged a tool result with a missing/
empty tool_call_id -- its orphan-detection set only ever collected
truthy ids, so an unpaired result with no id passed the final
chokepoint untouched.
Addresses teknium1's rebase request and wz-heng's review findings on
f0ac2c8f12