_resolve_child_fallback_chain re-implemented hermes_cli.fallback_config's entry
validation to log per-index warnings, wrapped a pure function in try/except, and
carried two names (routing_cfg / fallback_cfg) for one argument. It now delegates
to get_fallback_chain() and keeps the single warning for "no usable routes";
the facade re-export is dropped (import from the defining module). Tests collapse
to the parametrized decision table plus the pin-derivation, real-config-loader,
review-ownership and activation invariants (22 -> 20 cases, 417 -> ~230 lines).
A delegation.model-only pin (provider inherited) took the unpinned
column because pinned was keyed on override_provider alone, so the
child inherited the parent chain and a mid-run failure could silently
swap the pinned model. model at the call site is creds["model"] --
delegation.model config -- at both call sites, so keying on it adds no
false pins. base_url-only pins already resolve to a provider override.
Caught by review on the PR; two matrix tests added (model-only pin with
absent and declared chains).
Composes #80465's pin semantics with #80438/#80421's
delegation.fallback_providers semantics (#65038) and settles the
composition cell the three PRs leave undefined (#80450 map).
_resolve_child_fallback_chain implements the whole decision table in
one pure function: pinned children get no chain unless the delegation
section declares one (a delegation-scoped chain honors both explicit
intents; the silent drag in #80450 is specifically the PARENT chain
substituting a pin); an explicit empty list disables fallback; absent
config preserves historical parent inheritance exactly. Malformed
declared values log and fall back PIN-AWARE — never the parent chain on
a pinned child, so the config-error path cannot reintroduce the drag.
Entries normalize through the canonical get_fallback_chain.
The two existing inheritance tests are made hermetic against the real
user config (as #80438 also did).
kshitijk4poor