fix(anthropic): guard max_tokens against non-positive values

Port from openclaw/openclaw#66664. The build_anthropic_kwargs call site
used 'max_tokens or _get_anthropic_max_output(model)', which correctly
falls back when max_tokens is 0 or None (falsy) but lets negative ints
(-1, -500), fractional floats (0.5, 8192.7), NaN, and infinity leak
through to the Anthropic API. Anthropic rejects these with HTTP 400
('max_tokens: must be greater than or equal to 1'), turning a local
config error into a surprise mid-conversation failure.

Add two resolver helpers matching OpenClaw's:
  _resolve_positive_anthropic_max_tokens — returns int(value) only if
    value is a finite positive number; excludes bools, strings, NaN,
    infinity, sub-one positives (floor to 0).
  _resolve_anthropic_messages_max_tokens — prefers a positive requested
    value, else falls back to the model's output ceiling; raises
    ValueError only if no positive budget can be resolved.

The context-window clamp at the call site (max_tokens > context_length)
is preserved unchanged — it handles oversized values; the new resolver
handles non-positive values. These concerns are now cleanly separated.

Tests: 17 new cases covering positive/zero/negative ints, fractional
floats (both >1 and <1), NaN, infinity, booleans, strings, None, and
integration via build_anthropic_kwargs.

Refs: openclaw/openclaw#66664

agent/anthropic_adapter.py

@@ -117,6 +117,63 @@ def _get_anthropic_max_output(model: str) -> int:
     return best_val
 
 
+def _resolve_positive_anthropic_max_tokens(value) -> Optional[int]:
+    """Return ``value`` floored to a positive int, or ``None`` if it is not a
+    finite positive number. Ported from openclaw/openclaw#66664.
+
+    Anthropic's Messages API rejects ``max_tokens`` values that are 0,
+    negative, non-integer, or non-finite with HTTP 400. Python's ``or``
+    idiom (``max_tokens or fallback``) correctly catches ``0`` but lets
+    negative ints and fractional floats (``-1``, ``0.5``) through to the
+    API, producing a user-visible failure instead of a local error.
+    """
+    # Booleans are a subclass of int — exclude explicitly so ``True`` doesn't
+    # silently become 1 and ``False`` doesn't become 0.
+    if isinstance(value, bool):
+        return None
+    if not isinstance(value, (int, float)):
+        return None
+    try:
+        import math
+        if not math.isfinite(value):
+            return None
+    except Exception:
+        return None
+    floored = int(value)  # truncates toward zero for floats
+    return floored if floored > 0 else None
+
+
+def _resolve_anthropic_messages_max_tokens(
+    requested,
+    model: str,
+    context_length: Optional[int] = None,
+) -> int:
+    """Resolve the ``max_tokens`` budget for an Anthropic Messages call.
+
+    Prefers ``requested`` when it is a positive finite number; otherwise
+    falls back to the model's output ceiling. Raises ``ValueError`` if no
+    positive budget can be resolved (should not happen with current model
+    table defaults, but guards against a future regression where
+    ``_get_anthropic_max_output`` could return ``0``).
+
+    Separately, callers apply a context-window clamp — this resolver does
+    not, to keep the positive-value contract independent of endpoint
+    specifics.
+
+    Ported from openclaw/openclaw#66664 (resolveAnthropicMessagesMaxTokens).
+    """
+    resolved = _resolve_positive_anthropic_max_tokens(requested)
+    if resolved is not None:
+        return resolved
+    fallback = _get_anthropic_max_output(model)
+    if fallback > 0:
+        return fallback
+    raise ValueError(
+        f"Anthropic Messages adapter requires a positive max_tokens value for "
+        f"model {model!r}; got {requested!r} and no model default resolved."
+    )
+
+
 def _supports_adaptive_thinking(model: str) -> bool:
     """Return True for Claude 4.6+ models that support adaptive thinking."""
     return any(v in model for v in _ADAPTIVE_THINKING_SUBSTRINGS)
@@ -1391,7 +1448,12 @@ def build_anthropic_kwargs(
 
     model = normalize_model_name(model, preserve_dots=preserve_dots)
     # effective_max_tokens = output cap for this call (≠ total context window)
-    effective_max_tokens = max_tokens or _get_anthropic_max_output(model)
+    # Use the resolver helper so non-positive values (negative ints,
+    # fractional floats, NaN, non-numeric) fail locally with a clear error
+    # rather than 400-ing at the Anthropic API. See openclaw/openclaw#66664.
+    effective_max_tokens = _resolve_anthropic_messages_max_tokens(
+        max_tokens, model, context_length=context_length
+    )
 
     # Clamp output cap to fit inside the total context window.
     # Only matters for small custom endpoints where context_length < native