fix(shared): respect cached model context limits for Anthropic providers post-GA

After Anthropic's 1M context GA (2026-03-13), the beta header is no
longer sent. The existing detection relied solely on the beta header
to set anthropicContext1MEnabled, causing all Anthropic models to
fall back to the 200K default despite models.dev reporting 1M.

Update resolveActualContextLimit to check per-model cached limits
from provider config (populated from models.dev data) when the
explicit 1M flag is not set. Priority order:
1. Explicit 1M mode (beta header or env var) - all Anthropic models
2. Per-model cached limit from provider config
3. Default 200K fallback

This preserves the #2460 fix (explicit 1M flag always wins over
cached values) while allowing GA models to use their correct limits.

Fixes premature context warnings at 140K and unnecessary compaction
at 156K for opus-4-6 and sonnet-4-6 users without env var workaround.

src/shared/context-limit-resolver.ts

@@ -26,7 +26,13 @@ export function resolveActualContextLimit(
   modelCacheState?: ContextLimitModelCacheState,
 ): number | null {
   if (isAnthropicProvider(providerID)) {
-    return getAnthropicActualLimit(modelCacheState)
+    const explicit1M = getAnthropicActualLimit(modelCacheState)
+    if (explicit1M === 1_000_000) return explicit1M
+
+    const cachedLimit = modelCacheState?.modelContextLimitsCache?.get(`${providerID}/${modelID}`)
+    if (cachedLimit) return cachedLimit
+
+    return DEFAULT_ANTHROPIC_ACTUAL_LIMIT
   }
 
   return modelCacheState?.modelContextLimitsCache?.get(`${providerID}/${modelID}`) ?? null