[Bug] plan fails when a partition child is in a different schema from its parent

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Environment

• pgschema v1.12.3
• PostgreSQL 17.x (Supabase PostgreSQL image)
• plan uses an external PostgreSQL plan database via --plan-host
• Target schema is a Supabase application database with multiple application schemas

Problem

pgschema plan fails when a partitioned parent table and one of its partition children are in different schemas, even when the external plan database contains the relevant schemas and objects.

Observed structure:

• Partitioned parent: public.annotation_shapes
• Default partition: annotations_data.annotation_shapes_default

The desired SQL generated for the annotations_data schema contains a statement equivalent to:

CREATE TABLE IF NOT EXISTS annotation_shapes_default
PARTITION OF public.annotation_shapes (...) DEFAULT;

Planning the annotations_data schema fails while applying the desired SQL to the temporary plan schema:

partition "annotation_shapes_default" conflicts with existing default partition "annotation_shapes_default" (SQLSTATE 42P17)

Reproduction shape

1. Dump the child schema:

pgschema dump \
  --host <target-host> \
  --db <database> \
  --user <user> \
  --schema annotations_data \
  --qualify-schema > annotations_data.sql

2. Generate a plan for that schema using a dedicated external plan database:

pgschema plan \
  --host <target-host> \
  --db <database> \
  --user <user> \
  --schema annotations_data \
  --file annotations_data.sql \
  --plan-host <plan-host> \
  --plan-db <plan-db> \
  --plan-user <plan-user>

The failure occurs before a usable migration plan is produced. No apply was executed against the target database.

Expected behavior

The plan database should construct the desired state without creating a duplicate/conflicting partition, then produce a reviewable plan for the target schema.

Notes

This appears distinct from cross-schema foreign-key handling in #122, #284, and #548. The issue is specifically the interaction between per-schema planning and a partition child whose parent is in another schema.

The general partition fixes in #496/#498 and #499 are already included in v1.12.x, but they do not appear to cover this cross-schema parent/child case.