…M#5089)

Reverts the structural changes from QwenLM#5089 back to the pre-QwenLM#5089 shape:
AuthType stays a fixed enum (not `string`), the Protocol enum is removed,
modelProviders is `Record<authType, ModelConfig[]>` again (not
`{ protocol, models }`), and createContentGenerator dispatches on authType.
The v4->v5 settings migration is removed and SETTINGS_VERSION reverts to 4.

Features merged on top of QwenLM#5089 are kept and re-adapted to the old
enum+array structure (not reverted):
- QwenLM#5632 fastOnly/voiceOnly model flags (test fixtures reshaped to arrays)
- QwenLM#5638 workspace provider defaults (readProviderModels already tolerates
  both shapes; test fixtures reshaped to arrays)
- QwenLM#5729 active-runtime-model listing (pre-QwenLM#5089 getAllConfiguredModels
  already enumerates Object.values(AuthType), so the runtime model is
  included natively)
- QwenLM#5728 ACP set_config_option deterministic provider fixture (reshaped to
  array; the flake fix is preserved)

KNOWN DOWNGRADE CAVEAT: settings already migrated to $version:5 (shipped in
v0.19.0) retain the v5 `{ protocol, models }` modelProviders shape, which
the reverted ModelRegistry consumes as an array. Such settings will throw
on load until re-configured. A v5->v4 downgrade guard/migration is a
separate follow-up if backward compatibility for migrated users is needed.

#5745)

* revert(core): revert Protocol enum & model-identity decoupling (#5089)

Reverts the structural changes from #5089 back to the pre-#5089 shape:
AuthType stays a fixed enum (not `string`), the Protocol enum is removed,
modelProviders is `Record<authType, ModelConfig[]>` again (not
`{ protocol, models }`), and createContentGenerator dispatches on authType.
The v4->v5 settings migration is removed and SETTINGS_VERSION reverts to 4.

Features merged on top of #5089 are kept and re-adapted to the old
enum+array structure (not reverted):
- #5632 fastOnly/voiceOnly model flags (test fixtures reshaped to arrays)
- #5638 workspace provider defaults (readProviderModels already tolerates
  both shapes; test fixtures reshaped to arrays)
- #5729 active-runtime-model listing (pre-#5089 getAllConfiguredModels
  already enumerates Object.values(AuthType), so the runtime model is
  included natively)
- #5728 ACP set_config_option deterministic provider fixture (reshaped to
  array; the flake fix is preserved)

KNOWN DOWNGRADE CAVEAT: settings already migrated to $version:5 (shipped in
v0.19.0) retain the v5 `{ protocol, models }` modelProviders shape, which
the reverted ModelRegistry consumes as an array. Such settings will throw
on load until re-configured. A v5->v4 downgrade guard/migration is a
separate follow-up if backward compatibility for migrated users is needed.

* feat(cli): add v5->v4 settings downgrade migration for #5089 revert

After reverting #5089, settings already migrated to $version:5 (shipped in
v0.19.0) carry a modelProviders `{ protocol, models }` shape that the
reverted v4 readers consume as arrays, throwing "models is not iterable"
on load. This adds the inverse migration so those configs auto-converge to
v4 on load (the user-facing "automatically migrate $version:5 to 4").

- V5ToV4Migration: unwraps each modelProviders `{ protocol, models }` back
  to its `models` array, drops the now-implicit protocol (warning only when
  the explicit protocol differs from the key-derived one), and resets
  $version to 4.
- DOWNGRADE_MIGRATIONS keeps the downgrade out of the ascending forward
  ALL_MIGRATIONS chain (preserving its invariants); runMigrations and
  needsMigration consider both via a combined convergence set.
- needsMigration now gates on `=== SETTINGS_VERSION` instead of `>=`, so a
  newer-but-handled version (v5) is reported as needing migration while a
  genuinely unknown newer version (v6+) is still left untouched.

Covered by unit tests for the migration, the framework wiring, and an
end-to-end loadSettings downgrade-on-load test.

* fix(test): align integration settings-version constant with reverted v4

The integration suites hard-coded CURRENT_SETTINGS_VERSION = 5 (introduced
by #5676), which mismatched the reverted SETTINGS_VERSION = 4 and failed the
migration assertions ($version now writes 4, not 5). Revert the constant to
4 in both settings-migration and qwen-config-dir integration tests.

Verified: QWEN_SANDBOX=false vitest run --root ./integration-tests
cli/settings-migration.test.ts cli/qwen-config-dir.test.ts → 21 passed.

* fix: harden v5-era settings handling on the #5089 revert path

Addresses /qreview feedback on the revert:

- vscode findOpenaiModels: restore read-side tolerance for the V5
  { protocol, models } shape. The extension reads/writes settings.json
  without running the CLI v5->v4 migration, so a not-yet-downgraded
  $version:5 file would otherwise return [] and silently drop existing
  OpenAI models on the next write. (Critical)
- modelRegistry.registerAuthTypeModels: guard against a non-array provider
  value (skip + warn) instead of throwing an opaque "models is not
  iterable" — covers hand-edited or unmigrated files the downgrade misses.
- needsMigration JSDoc: update the stale ">= SETTINGS_VERSION" wording to
  match the "=== SETTINGS_VERSION, else fall through" logic the downgrade
  path depends on.
- settings.test.ts: also assert the v5->v4 downgrade is persisted to disk
  (.tmp write-back), not just the in-memory merged result.

Adds tests for the registry guard and the vscode V5 read tolerance.

* fix(core): break contentGenerator import cycle + cover reverted error paths

Addresses /review suggestions on the revert:

- contentGenerator: import PROVIDER_SOURCED_FIELDS from constants.js (where
  it is actually defined) instead of modelsConfig.js, breaking the runtime
  import cycle contentGenerator -> modelsConfig -> contentGenerator.
  constants.js only references contentGenerator at the type level, which is
  erased at runtime, so no cycle remains.
- contentGenerator.test: add coverage for the two authType error paths the
  revert restored (missing authType -> "must have an authType"; unknown
  authType -> "Unsupported authType"), which #5089's protocol-based tests
  had replaced. Neither was covered before.

The acpAgent z.nativeEnum(AuthType).parse(methodId) suggestion is left as-is:
that line is byte-identical to pre-#5089, so it is pre-existing behavior the
revert faithfully restores rather than a regression of this PR.