fix(anthropic): smart thinking block signature management (#6112)

Anthropic signs thinking blocks against the full turn content. Any
upstream mutation (context compression, session truncation, orphan
stripping, message merging) invalidates the signature, causing HTTP 400
'Invalid signature in thinking block' — especially in long-lived
gateway sessions.

Strategy (following clawdbot/OpenClaw pattern):

1. Strip thinking/redacted_thinking from all assistant messages EXCEPT
   the last one — preserves reasoning continuity on the current
   tool-use chain while avoiding stale signature errors on older turns.

2. Downgrade unsigned thinking blocks to plain text — Anthropic can't
   validate them, but the reasoning content is preserved.

3. Strip cache_control from thinking/redacted_thinking blocks to
   prevent cache markers from interfering with signature validation.

4. Drop thinking blocks from the second message when merging
   consecutive assistant messages (role alternation enforcement).

5. Error recovery: on HTTP 400 mentioning 'signature' and 'thinking',
   strip all reasoning_details from the conversation and retry once.
   This is the safety net for edge cases the proactive stripping
   misses.

Addresses the issue reported in PR #6086 by @mingginwan while
preserving reasoning continuity (their PR stripped ALL thinking
blocks unconditionally).

Files changed:
- agent/anthropic_adapter.py: thinking block management in
  convert_messages_to_anthropic (strip old turns, downgrade unsigned,
  strip cache_control, merge-time strip)
- run_agent.py: one-shot signature error recovery in retry loop
- tests/test_anthropic_adapter.py: 10 new tests covering all cases

run_agent.py

@@ -7288,6 +7288,7 @@ class AIAgent:
             codex_auth_retry_attempted=False
             anthropic_auth_retry_attempted=False
             nous_auth_retry_attempted=False
+            thinking_sig_retry_attempted = False
             has_retried_429 = False
             restart_with_compressed_messages = False
             restart_with_length_continuation = False
@@ -7877,8 +7878,38 @@ class AIAgent:
                         print(f"{self.log_prefix}     • Check ANTHROPIC_API_KEY in {_dhh}/.env for API keys or legacy token values")
                         print(f"{self.log_prefix}     • For API keys: verify at https://console.anthropic.com/settings/keys")
                         print(f"{self.log_prefix}     • For Claude Code: run 'claude /login' to refresh, then retry")
-                        print(f"{self.log_prefix}     • Clear stale keys: hermes config set ANTHROPIC_TOKEN \"\"")
-                        print(f"{self.log_prefix}     • Legacy cleanup: hermes config set ANTHROPIC_API_KEY \"\"")
+                        print(f"{self.log_prefix}     • Legacy cleanup: hermes config set ANTHROPIC_TOKEN \"\"")
+                        print(f"{self.log_prefix}     • Clear stale keys: hermes config set ANTHROPIC_API_KEY \"\"")
+
+                    # ── Thinking block signature recovery ─────────────────
+                    # Anthropic signs thinking blocks against the full turn
+                    # content.  Any upstream mutation (context compression,
+                    # session truncation, message merging) invalidates the
+                    # signature → HTTP 400.  Recovery: strip reasoning_details
+                    # from all messages so the next retry sends no thinking
+                    # blocks at all.  One-shot — don't retry infinitely.
+                    if (
+                        self.api_mode == "anthropic_messages"
+                        and status_code == 400
+                        and not thinking_sig_retry_attempted
+                    ):
+                        _err_msg_lower = str(api_error).lower()
+                        if "signature" in _err_msg_lower and "thinking" in _err_msg_lower:
+                            thinking_sig_retry_attempted = True
+                            for _m in messages:
+                                if isinstance(_m, dict):
+                                    _m.pop("reasoning_details", None)
+                            self._vprint(
+                                f"{self.log_prefix}⚠️  Thinking block signature invalid — "
+                                f"stripped all thinking blocks, retrying...",
+                                force=True,
+                            )
+                            logging.warning(
+                                "%sThinking block signature recovery: stripped "
+                                "reasoning_details from %d messages",
+                                self.log_prefix, len(messages),
+                            )
+                            continue
 
                     retry_count += 1
                     elapsed_time = time.time() - api_start_time