fix(config): distinguish disabled-plugin toolsets from typos in validation - #56731

What changed

validate_platform_toolsets() (hermes_cli/toolset_validation.py, added for #38798) now accepts an optional third argument, known_plugin_toolsets, and cross-references it against unknown toolset names to produce a more accurate warning when the real cause is a disabled/uninstalled plugin rather than a typo.

Why

Hermes tracks toolset names across three separate places:

• plugins.enabled — which plugins are turned on
• known_plugin_toolsets — per-platform snapshot of plugin toolset keys seen the last time hermes tools saved that platform (hermes_cli/tools_config.py::_save_platform_tools)
• platform_toolsets — the actual per-platform toolset selection

validate_platform_toolsets already does excellent work catching corrupted/renamed entries (the #38798 incident: hermes-cli silently rewritten to the nonexistent hermes), including a hermes-<platform> "did you mean" guess. But that guess is actively misleading for a very common real-world case: a platform_toolsets entry that names a real plugin toolset whose plugin is now disabled (or its package uninstalled). Since is_valid_toolset (backed by the live toolset registry) only knows about currently-loaded plugins, a disabled plugin's toolset name looks identical to a typo to the existing check — same generic "unknown toolset ''" warning, with no hermes-<platform> hint either (since that guess doesn't happen to match), leaving the real cause unstated.

known_plugin_toolsets already has exactly the information needed to tell these apart: if a name is in known_plugin_toolsets[platform], it was a real, valid plugin toolset the last time hermes tools ran for that platform — so its current invalidity almost certainly means the plugin got disabled or uninstalled, not that the name was ever wrong.

Approach

• New optional known_plugin_toolsets param, default None (or any non-dict) → identical behavior to before this change, so existing callers/tests aren't broken.
• When a name fails is_valid_toolset AND is found in known_plugin_toolsets[platform], emit a distinct, more actionable warning pointing at plugins.enabled and plugin installation, instead of the generic unknown-toolset message with its (here, irrelevant) hermes-<platform> guess.
• Threaded through the one existing call site in hermes_cli/config.py (read_raw_config().get("known_plugin_toolsets")), reusing the same try/except-wrapped, best-effort validation pass that already runs after every config migration.
• No new call sites, no new validation pass — purely widening the existing one.

Test plan

• Extended tests/hermes_cli/test_toolset_validation.py with:
  • a disabled-plugin case producing the new plugin-specific warning (and confirming it does not also carry the typo-guess wording)
  • a case where the name is known for a different platform, confirming it still falls back to the generic unknown-toolset warning (the cross-check is platform-scoped)
  • omitted/malformed known_plugin_toolsets (None, [], a string, {} with a non-list value) all falling back to the pre-existing generic behavior
• Ran scripts/run_tests.sh tests/hermes_cli/test_toolset_validation.py tests/hermes_cli/test_config.py — 153 tests, all passing, no regressions in the config-migration suite that exercises this call site.