🐾 fix(mantle): stop installing the unproven 20k max_tokens clamp

The fine-grained-tool-streaming opt-out is what actually fixes the
"Streaming completed but tool call never finished" crash loop: under
that beta an output cutoff mid-tool_use ends the stream without
content_block_stop, fast-agent's tool tracker leaves the block open,
and _raise_for_incomplete_anthropic_tools raises a RuntimeError that
bypasses the graceful stop_reason=max_tokens path and burns the retry
ladder. That shim costs no output length and is kept.

The companion max_tokens clamp is not. Its 20 000 ceiling was an
empirical observation, never a documented Mantle limit, and
re-investigation could not establish what enforces it: fast-agent
carries no 20 000 default anywhere (the matching TASK_BUDGET_MIN_TOKENS
is a validation floor for a different, unconfigured feature), no model
overlay is configured, and ModelDatabase reports
max_output_tokens=128000 for opus-4-8. Installing it would cement a
ceiling we cannot prove and silently truncate turns that might
otherwise complete.

install_max_tokens_clamp() is left in place, unwired, so it can be
re-enabled if the limit is ever confirmed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/test_mantle_shims.py

@@ -200,7 +200,9 @@ def test_maybe_install_installs_when_mantle(monkeypatch: pytest.MonkeyPatch) ->
 
     installed = mantle_shims.maybe_install("https://bedrock-mantle.us-east-1.api.aws/anthropic")
     assert installed is True
-    assert calls == ["wire", "tool_use", "beta_opt_out", "max_tokens"]
+    # "max_tokens" is intentionally absent: the 20 000 clamp is not installed
+    # by default because the ceiling was never confirmed. See install_all().
+    assert calls == ["wire", "tool_use", "beta_opt_out"]
 
 
 def test_maybe_install_noop_for_non_mantle(monkeypatch: pytest.MonkeyPatch) -> None: