Find a cleaner solution for the return_skipped_upsert? logic introduced in #626

[barnabasJ]

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Is your feature request related to a problem? Please describe.

#626 adds support for returning skipped records from bulk_upserts. but we need to do an extra query and some data wrangling to achieve it.

Describe the solution you'd like

I tried doing it as part of the upsert statement by adding the upsert_condition logic to the update statement and add a marker to the return values to check if one of the inserts was actually skipped because of the condition logic.

1. CASE WHEN in DO UPDATE SET was successfully implemented - the conditional logic works, SQL generates correctly
2. The goal was to add the condition to RETURNING like:
   RETURNING id, price, (price != EXCLUDED.price) AS met_upsert_condition
3. PostgreSQL supports this - expressions in RETURNING are valid SQL
4. But Ecto doesn't expose a way to do it:
   - The :returning option flows through quote_names() which calls quote_name/1 on each item
   - quote_name/1 only has function clauses for atoms and strings - it quotes them as identifiers
   - If {:unsafe_fragment, "..."} is passed (like :conflict_target supports), it crashes with FunctionClauseError because there's no matching clause
   - If a plain string like "(condition) AS met" is passed, it gets quoted as "(condition) AS met" which is an invalid identifier

Describe alternatives you've considered

No response

Additional context

No response