Addresses egilewski (Codex) CR on PR #52355: the persisted-row fallback in
_resume_target_allowed() skipped the platform/source check when sessions.source
was blank (the row_src guard only rejects a *mismatching* non-blank source),
then accepted the row on user_id equality alone. A legacy/malformed row with a
blank source but a matching user_id was therefore resumable — an identified
caller could bind to a transcript whose origin it can't prove.
Now an identity-bearing caller is allowed only when the row proves BOTH the
same owner (non-blank user_id match) AND the same platform/origin (non-blank
source match). A blank/legacy source fails closed, exactly like a missing
user_id. No-identity (single-user) callers are unaffected.
Adds a regression replaying the reviewer's blank-source same-uid probe.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
claudlos
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