fix(anthropic): don't fall back to raw content when all blocks filtered

Follow-up per independent review of #68633 (GPT-5.6-sol-xhigh in Codex,
reviewer egilewski) on this PR.

Two real bugs in the blank-text-block filtering added by that fix:

1. `effective = blocks or content` fell back to the RAW, unfiltered
   `content` variable whenever every block was filtered out as blank --
   which happens precisely when the entire message content WAS the
   blank/whitespace payload the filter exists to remove (a sole blank
   text block, a sole cache-marked blank block, or standalone
   whitespace scalar content with no tool_calls). The fallback silently
   restored the exact invalid content the filtering just stripped,
   leaving the message provider-invalid.

   Fixed: `effective = blocks if blocks else [{"type": "text", "text":
   "(empty)"}]` -- never falls back to raw `content`. Also moved the
   cache_control application (both the relocated-from-a-dropped-block
   marker and the message-level marker) to run against `effective`
   instead of the pre-fallback `blocks`, so a cache marker on a block
   that was the ONLY content still lands on the (empty) placeholder
   rather than being silently lost when `blocks` was empty at the
   point it would otherwise have been applied.

2. The normal-path blank-text check used `(blk.get("text") or "").strip()`,
   which is not type-safe for a truthy NON-string, non-None text value
   (e.g. an int or dict from an invalid upstream payload) -- `or`
   doesn't substitute for a truthy value, so `(7 or "").strip()` still
   raises AttributeError. Now checks `isinstance(text, str)` first,
   matching the replay path's `_sanitize_replay_block()`, which the
   reviewer confirmed was already correctly type-safe.

Added regression tests for: sole blank list block, sole whitespace
scalar content, sole cache-marked blank block (marker relocation to
the placeholder), a truthy non-string (int) text value both mixed with
a surviving tool_use and as the sole content, and a dict-valued text
field. 7/7 new tests pass; 193/193 in the full
tests/agent/test_anthropic_adapter.py file; 23/23 in
tests/agent/test_prompt_caching.py (unaffected, confirmed).

agent/anthropic_adapter.py

@@ -2061,15 +2061,18 @@ def _convert_assistant_message(m: Dict[str, Any]) -> Dict[str, Any]:
                 # Bedrock and strict Anthropic-compatible endpoints reject
                 # text blocks where "text" is empty or whitespace-only. The
                 # ordered-replay path enforces the same invariant via
-                # _sanitize_replay_block(). Type-safe: input text blocks can
-                # carry text=None (_convert_content_part_to_anthropic
-                # preserves it from an invalid upstream payload), which a
-                # bare .strip() would crash on.
+                # _sanitize_replay_block(). Type-safe against ANY invalid
+                # "text" value from an upstream payload -- None, or a
+                # truthy non-string like an int -- not just None: checking
+                # isinstance() first (rather than `blk.get("text") or ""`)
+                # means a non-string value is treated as blank/invalid
+                # instead of reaching .strip() and raising AttributeError.
                 for blk in converted_content:
+                    _blk_text = blk.get("text") if isinstance(blk, dict) else None
                     if (
                         isinstance(blk, dict)
                         and blk.get("type") == "text"
-                        and not (blk.get("text") or "").strip()
+                        and (not isinstance(_blk_text, str) or not _blk_text.strip())
                     ):
                         if isinstance(blk.get("cache_control"), dict):
                             _relocated_cache_control = blk["cache_control"]
@@ -2097,13 +2100,6 @@ def _convert_assistant_message(m: Dict[str, Any]) -> Dict[str, Any]:
             "name": fn.get("name", ""),
             "input": parsed_args,
         })
-    if _relocated_cache_control is not None:
-        _apply_assistant_cache_control_to_last_cacheable_block(
-            blocks, _relocated_cache_control
-        )
-    _apply_assistant_cache_control_to_last_cacheable_block(
-        blocks, m.get("cache_control")
-    )
     # Kimi's /coding endpoint (Anthropic protocol) requires assistant
     # tool-call messages to carry reasoning_content when thinking is
     # enabled server-side.  Preserve it as a thinking block so Kimi
@@ -2129,19 +2125,26 @@ def _convert_assistant_message(m: Dict[str, Any]) -> Dict[str, Any]:
     )
     if isinstance(reasoning_content, str) and not _already_has_thinking:
         blocks.insert(0, {"type": "thinking", "thinking": reasoning_content})
-    # Anthropic rejects empty assistant content
-    effective = blocks or content
-    if not effective or effective == "":
-        effective = [{"type": "text", "text": _EMPTY_TEXT_PLACEHOLDER}]
-    elif isinstance(effective, list):
-        # The all-empty guard above misses a list that still contains a
-        # whitespace-only text block (e.g. from a content array of blank parts,
-        # or compression). Those also trip "text content blocks must contain
-        # non-whitespace text" (#69512). Coerce text blocks in place; other
-        # block types (thinking/tool_use/image) are left untouched.
-        for blk in effective:
-            if isinstance(blk, dict) and blk.get("type") == "text":
-                blk["text"] = _safe_text(blk.get("text", ""))
+    # Anthropic rejects empty assistant content. IMPORTANT: fall back only
+    # to the placeholder, never to the raw `content` variable -- `content`
+    # is the UNFILTERED original message content, and can itself be exactly
+    # the blank/whitespace-only payload the filtering above just removed
+    # (a sole blank text block, or scalar whitespace with no tool_calls).
+    # `blocks or content` there would silently restore the invalid provider
+    # payload this function exists to prevent (#69512).
+    effective = blocks if blocks else [{"type": "text", "text": _EMPTY_TEXT_PLACEHOLDER}]
+    # Applied here (after the empty-fallback resolution) rather than
+    # earlier against `blocks` directly, so a cache_control relocated from
+    # a dropped blank block that was the ONLY block still lands on the
+    # (empty) placeholder instead of being silently lost when blocks was
+    # empty at the point the marker would otherwise have been applied.
+    if _relocated_cache_control is not None:
+        _apply_assistant_cache_control_to_last_cacheable_block(
+            effective, _relocated_cache_control
+        )
+    _apply_assistant_cache_control_to_last_cacheable_block(
+        effective, m.get("cache_control")
+    )
     return {"role": "assistant", "content": effective}