fix(anthropic): use model-native output limits instead of hardcoded 16K (#3426)

The Anthropic adapter defaulted to max_tokens=16384 when no explicit value
was configured.  This severely limits thinking-enabled models where thinking
tokens count toward max_tokens:

- Claude Opus 4.6 supports 128K output but was capped at 16K
- Claude Sonnet 4.6 supports 64K output but was capped at 16K

With extended thinking (adaptive or budget-based), the model could exhaust
the entire 16K on reasoning, leaving zero tokens for the actual response.
This caused two user-visible errors:
- 'Response truncated (finish_reason=length)' — thinking consumed most tokens
- 'Response only contains think block with no content' — thinking consumed all

Fix: add _ANTHROPIC_OUTPUT_LIMITS lookup table (sourced from Anthropic docs
and Cline's model catalog) and use the model's actual output limit as the
default.  Unknown future models default to 128K (the current maximum).

Also adds context_length clamping: if the user configured a smaller context
window (e.g. custom endpoint), max_tokens is clamped to context_length - 1
to avoid exceeding the window.

Closes #2706

run_agent.py

@@ -4378,6 +4378,10 @@ class AIAgent:
         if self.api_mode == "anthropic_messages":
             from agent.anthropic_adapter import build_anthropic_kwargs
             anthropic_messages = self._prepare_anthropic_messages_for_api(api_messages)
+            # Pass context_length so the adapter can clamp max_tokens if the
+            # user configured a smaller context window than the model's output limit.
+            ctx_len = getattr(self, "context_compressor", None)
+            ctx_len = ctx_len.context_length if ctx_len else None
             return build_anthropic_kwargs(
                 model=self.model,
                 messages=anthropic_messages,
@@ -4386,6 +4390,7 @@ class AIAgent:
                 reasoning_config=self.reasoning_config,
                 is_oauth=self._is_anthropic_oauth,
                 preserve_dots=self._anthropic_preserve_dots(),
+                context_length=ctx_len,
             )
 
         if self.api_mode == "codex_responses":