Calvin RETURNING swallows shaper errors; a failed row encode answers an ERROR tag

[EnRaiha]

Three sites replace a shaper failure instead of surfacing it. Verified on a89b56ddb.

Sites

Site	Behaviour
pgwire/handler/routing/calvin_response.rs:74	&& let Ok(ShapeOutcome::Rows(shaped)) = … — an Err falls through to the command-tag/affected-count path, so a RETURNING payload that failed to shape reports a successful statement with no rows
native/dispatch/conversion.rs:229	same && let Ok(..) swallow; the response degrades to a count-only result
pgwire/handler/plan.rs:207-210	an encode_field failure logs and answers an ERROR command tag, replacing the error

Should

• Calvin pgwire: raise the error through shape_error_to_pg (the routed-path helper) with the error's own SQLSTATE.
• Native apply: raise through shape_error_to_native.
• plan.rs: ToSqlText for &str is total in pgwire 0.41, so the encode failure cannot happen — remove the dead fallback instead of keeping a branch that reads as a swallow.

Evidence

• The two let Ok(ShapeOutcome) sites are the only shaper-error swallows left after the routed-site fix (repo-wide grep).
• A local fix with the surrounding suites green exists and will be proposed with Closes to this issue.