A config migration (or hand-edit) that leaves an invalid toolset name in
`platform_toolsets` — e.g. the #38798 corruption that rewrote `hermes-cli` to
the non-existent `hermes` — silently disabled all affected tools:
resolve_toolset() returns [] for an unknown name, so the agent quietly lost its
tools with no error, warning, or log entry and degraded to text-only replies.
Surface it loudly at two points:
- After migration (migrate_config): validate platform_toolsets and record/print
a warning per unknown name, with a `hermes-<platform>` suggestion when that
would have been valid (the exact #38798 shape).
- At runtime (_get_platform_tools): if a platform was explicitly configured but
every toolset name is invalid, log a warning when tools are resolved for a
session — so an ALREADY-corrupted config is caught at startup, not only on the
next `hermes update`.
Logic lives in a new pure, side-effect-free helper (toolset_validation.py) with
validate_toolset injected, so it is unit-testable without the tool registry.
Note: the original v25→v26 migration that caused the corruption no longer
exists (config format is now v30; no migration step rewrites toolset names).
This change is the durable defense against the silent-failure mode regardless
of cause, matching the issue's "Expected: log a warning".
Salvaged from #39207 by @lEWFkRAD (authorship preserved via cherry-pick).
Tests: 9 helper cases (incl. the #38798 corruption shape, mixed valid/invalid,
zero-tools state, non-dict/scalar/non-string) + a runtime caplog test — both the
helper warning and the runtime guard mutation-verified to fail without the fix.
Closes#38798. Supersedes #39581 (prevent-in-v25→v26 — that path is gone),
#41006 / #40208 (repair-migration for already-corrupted configs).
