fix: shrink images to reported provider dimension limit (#45979)

Parse provider-reported image pixel ceilings so many-image Anthropic requests can recover by shrinking Retina screenshots below the stricter limit instead of retrying the same rejected payload.

agent/conversation_compression.py

@@ -631,7 +631,11 @@ def compress_context(
     return compressed, new_system_prompt
 
 
-def try_shrink_image_parts_in_messages(api_messages: list) -> bool:
+def try_shrink_image_parts_in_messages(
+    api_messages: list,
+    *,
+    max_dimension: int = 8000,
+) -> bool:
     """Re-encode all native image parts at a smaller size to recover from
     image-too-large errors (Anthropic 5 MB, unknown other providers).
 
@@ -642,7 +646,8 @@ def try_shrink_image_parts_in_messages(api_messages: list) -> bool:
     Strategy: look for ``image_url`` / ``input_image`` parts carrying a
     ``data:image/...;base64,...`` payload.  For each one whose encoded
     size exceeds 4 MB (a safe target that slides under Anthropic's 5 MB
-    ceiling with header overhead), write the base64 to a tempfile, call
+    ceiling with header overhead) or whose longest side exceeds
+    ``max_dimension``, write the base64 to a tempfile, call
     ``vision_tools._resize_image_for_vision`` to produce a smaller data
     URL, and substitute it in place.
 
@@ -664,10 +669,9 @@ def try_shrink_image_parts_in_messages(api_messages: list) -> bool:
     # after a confirmed provider rejection, so the alternative is failure.
     target_bytes = 4 * 1024 * 1024
     # Anthropic enforces an 8000px per-side dimension cap independently of
-    # the 5 MB byte cap.  A tall screenshot can be well under 5 MB yet far
-    # over 8000px (e.g. 1200×12000 at 0.06 MB).  We check pixel dimensions
-    # even when the byte budget is fine.
-    max_dimension = 8000
+    # the 5 MB byte cap.  In many-image requests, the provider can report a
+    # lower cap (observed: 2000px).  The caller passes that parsed ceiling
+    # when the rejection includes it.
     changed_count = 0
     # Track parts that are over the target but could NOT be shrunk under it.
     # If any survive, retrying is pointless — the same oversized payload will
@@ -684,9 +688,9 @@ def try_shrink_image_parts_in_messages(api_messages: list) -> bool:
         # Check both byte size AND pixel dimensions.
         needs_shrink = len(url) > target_bytes  # over byte budget
         if not needs_shrink:
-            # Even if bytes are fine, check pixel dimensions against
-            # Anthropic's 8000px cap.  A tall image can be tiny in bytes
-            # yet huge in pixels.
+            # Even if bytes are fine, check pixel dimensions against the
+            # provider's reported per-side cap.  A screenshot can be tiny in
+            # bytes yet too large in pixels.
             try:
                 import base64 as _b64_dim
                 header_d, _, data_d = url.partition(",")