fix(model): preserve LM Studio '@' quant suffixes in model name resolution

stripModelProfileSuffix() in providers.ts naively truncated model names at
the first '@', discarding quant variants like @iq3_xxs, @iq4_xs, @q4_k_xl
that LM Studio uses to distinguish quantization levels.

This caused two user-facing bugs (fixes #71474):
1. /model lmstudio/qwen3.6-27b@iq3_xxs → 'model not allowed: lmstudio/qwen3.6-27b'
2. API requests sent truncated model name → LM Studio picked a random quant

Changes:
- Replace the naive indexOf('@') strip in providers.ts with
  splitTrailingAuthProfile() which already handles quant suffixes
- Extend the quant-suffix regex (q\d+...) to also match importance-
  quantization tags (iq3_xxs, iq4_xs, ...) via i?q\d+ pattern
- Add tests for @iq* quant suffixes and auth-profile-after-iq combos

src/plugins/providers.ts

@@ -1,3 +1,4 @@
+import { splitTrailingAuthProfile } from "../agents/model-ref-profile.js";
 import { normalizeProviderId } from "../agents/provider-id.js";
 import { withBundledPluginVitestCompat } from "./bundled-compat.js";
 import { resolveEffectivePluginActivationState } from "./config-state.js";
@@ -353,9 +354,7 @@ function resolveManifestRegistry(params: {
 }
 
 function stripModelProfileSuffix(value: string): string {
-  const trimmed = value.trim();
-  const at = trimmed.indexOf("@");
-  return at <= 0 ? trimmed : trimmed.slice(0, at).trim();
+  return splitTrailingAuthProfile(value).model;
 }
 
 function splitExplicitModelRef(rawModel: string): { provider?: string; modelId: string } | null {