Make native session ID lookup source-qualified and ambiguity-safe

[andrei-hasna]

Problem

Exact lookup currently uses id OR source_id and may return an arbitrary row. Once Codex and Codewith are separate sources, the same native source ID can legitimately occur in both.

Expected

Support a source-qualified identifier such as codewith: and/or --source codewith.

Acceptance

• Internal database IDs remain exact.
• Source-qualified native IDs resolve exactly one row.
• Unqualified duplicate source IDs return an explicit ambiguity error.
• Prefix lookup remains deterministic.
• Tests cover exact and prefix collisions.

Todo: OPE2-00077