Open OIDC child session when assuming organization

OIDC login dropped organization_id before the provider redirect, so
callbacks with an existing matching root session never created an org
child session. Persist organization_id in OIDC state, open the child
session on callback, and forward the parameter from the sign-in UI.

Signed-off-by: Bryan Frimin <bryan@probo.com>

pkg/coredata/migrations/20260617T120000Z.sql

@@ -0,0 +1,15 @@
+-- Copyright (c) 2026 Probo Inc <hello@probo.com>.
+--
+-- Permission to use, copy, modify, and/or distribute this software for any
+-- purpose with or without fee is hereby granted, provided that the above
+-- copyright notice and this permission notice appear in all copies.
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+-- REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
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+-- PERFORMANCE OF THIS SOFTWARE.
+
+ALTER TABLE iam_oidc_states ADD COLUMN organization_id TEXT;