Refresh context-length zero-guard on current upstream/main

Reapply the non-positive context-length guards onto the post-history-replacement
mainline without carrying any stale branch history. save_context_length() now
refuses to persist length <= 0 (keeping upstream's normalized _context_cache_key),
and get_model_context_length() drops non-positive cache hits at the head of the
invalidation chain (Codex/Kimi/MiniMax/Grok branches become elif) so a poisoned
entry re-resolves instead of short-circuiting to 0.

Refresh of PR #25812; original head d62ed5eb92f057d8c707ba937b44f168f2df0677.

agent/model_metadata.py

@@ -1452,6 +1452,15 @@ def save_context_length(model: str, base_url: str, length: int) -> None:
     Cache key is ``model@base_url`` so the same model name served from
     different providers can have different limits.
     """
+    # Never persist non-positive values — a 0 or negative context length
+    # is always a bug and would poison the cache, causing downstream
+    # `get_model_context_length()` to return 0 (since `0 is not None`).
+    if length <= 0:
+        logger.warning(
+            "Refusing to cache non-positive context length %s -> %s tokens",
+            f"{model}@{base_url}", length,
+        )
+        return
     key = _context_cache_key(model, base_url)
     cache = _load_context_cache()
     if cache.get(key) == length:
@@ -2664,8 +2673,19 @@ def get_model_context_length(
     if base_url and not _skip_persistent_context_cache(base_url, provider):
         cached = get_cached_context_length(model, base_url)
         if cached is not None:
+            # Reject non-positive cached values — a 0 or negative value
+            # is always a bug (corrupted cache, probe failure, or manual
+            # edit).  Without this guard, `0 is not None` short-circuits
+            # the resolution chain and the compressor gets context_length=0,
+            # breaking every status-bar and /usage display downstream.
+            if cached <= 0:
+                logger.warning(
+                    "Dropping non-positive cache entry %s@%s -> %s; re-resolving",
+                    model, base_url, cached,
+                )
+                _invalidate_cached_context_length(model, base_url)
             # Invalidate stale 32k cache entries for Kimi-family models.
-            if cached <= 32768 and _model_name_suggests_kimi(model):
+            elif cached <= 32768 and _model_name_suggests_kimi(model):
                 logger.info(
                     "Dropping stale Kimi cache entry %s@%s -> %s (OpenRouter underreport); "
                     "re-resolving via hardcoded defaults",