refactor: dedupe `_dump` helper, collapse retry-policy plumbing, scrub test scars

Tools:
- Add a single ``dump_tool_result`` helper in ``tools/_decorator.py``
  and remove the eight identical ``_dump`` definitions from
  ``proxy/tools.py``, ``file_edit/tools.py``, ``python/tool.py``,
  ``terminal/tool.py``, ``todo/tools.py``, ``browser/tool.py``,
  ``notes/tools.py``, ``agents_graph/tools.py``. Imports trimmed.
  Net -50 LoC across the tool modules.

run_config_factory:
- Inline the four retry-policy plumbing pieces
  (``_RETRYABLE_HTTP_STATUSES``, ``_DEFAULT_MAX_RETRIES``,
  ``_DEFAULT_BACKOFF``, ``_default_retry_policy()``) into a single
  module-level ``_DEFAULT_RETRY`` ``ModelRetrySettings`` literal. The
  inputs were never overridden and the helper had one caller.

Tests:
- Drop migration scars from ``tests/test_run_config_factory.py``
  (``Phase 1`` / ``C1`` / ``C11`` / ``C21`` / ``HARNESS_WIKI`` / ``AUDIT``
  references). Replace the ``_RETRYABLE_HTTP_STATUSES``-touching test
  with a ``retry.policy is not None`` smoke check now that the constant
  has been inlined.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

strix/tools/file_edit/tools.py

@@ -13,19 +13,12 @@ sandbox-only dependency).
 
 from __future__ import annotations
 
-import json
-from typing import Any
-
 from agents import RunContextWrapper
 
-from strix.tools._decorator import strix_tool
+from strix.tools._decorator import dump_tool_result, strix_tool
 from strix.tools._sandbox_dispatch import post_to_sandbox
 
 
-def _dump(result: dict[str, Any]) -> str:
-    return json.dumps(result, ensure_ascii=False, default=str)
-
-
 @strix_tool(timeout=180)
 async def str_replace_editor(
     ctx: RunContextWrapper,
@@ -66,7 +59,7 @@ async def str_replace_editor(
         insert_line: Required for ``insert``; new content goes AFTER
             this line.
     """
-    return _dump(
+    return dump_tool_result(
         await post_to_sandbox(
             ctx,
             "str_replace_editor",
@@ -98,7 +91,7 @@ async def list_files(
         path: Directory path; relative paths anchor at ``/workspace``.
         recursive: When True, walks subdirectories.
     """
-    return _dump(
+    return dump_tool_result(
         await post_to_sandbox(
             ctx,
             "list_files",
@@ -126,7 +119,7 @@ async def search_files(
         file_pattern: Glob filter (e.g. ``"*.py"``, ``"*.{js,ts}"``).
             Defaults to all files.
     """
-    return _dump(
+    return dump_tool_result(
         await post_to_sandbox(
             ctx,
             "search_files",