refactor(mcp_client): migrate 3 nested helper BC sites to Result-drain (Phase 8)

Three nested helper functions inside _result variants had silent-swallow
or broad-catch patterns that the audit still flagged:

1. py_find_usages_result._search_file (L846):
   Was: 'try/except Exception: pass' (silent-swallow per-file read errors)
   Now: try/except (OSError, UnicodeDecodeError) as e: errors.append(ErrorInfo(...))
   Errors propagated via the parent's Result.errors

2. derive_code_path_result (L957):
   Was: 'try/except Exception: continue' (silent-swallow file parse errors)
   Now: try/except (SyntaxError, ValueError) as e: file_errors.append(ErrorInfo(...))
   Errors propagated via the parent's Result.errors

3. derive_code_path_result._trace (L996):
   Was: try/except Exception as e: output.append(f-string with error)
   Now: same output.append + ALSO appends ErrorInfo to file_errors
   Drain: output appears in the result data string (operator-visible)

All 3 sites now comply with the data-oriented convention.

Audit: mcp_client migration-target sites: 0 (was 3). Categories:
  BOUNDARY_CONVERSION: 5, INTERNAL_COMPLIANT: 43

src/mcp_client.py

@@ -832,7 +832,8 @@ def py_find_usages_result(path: str, name: str) -> Result[str]:
  try:
   import re
   pattern = re.compile(r"\b" + re.escape(name) + r"\b")
-  results = []
+  results: list[str] = []
+  file_errors: list[ErrorInfo] = []
   def _search_file(fp: Path) -> None:
    if fp.name == "history.toml" or fp.name.endswith("_history.toml"): return
    if not _is_allowed(fp): return
@@ -843,8 +844,8 @@ def py_find_usages_result(path: str, name: str) -> Result[str]:
      if pattern.search(line):
       rel = fp.relative_to(_primary_base_dir if _primary_base_dir else Path.cwd())
       results.append(f"{rel}:{i}: {line.strip()[:100]}")
-   except Exception:
-    pass
+   except (OSError, UnicodeDecodeError) as e:
+    errors.append(ErrorInfo(kind=ErrorKind.INTERNAL, message=f"failed to read {fp}: {e}", source="mcp.py_find_usages_result._search_file", original=e))
   if p.is_file():
    _search_file(p)
   else:
@@ -854,10 +855,12 @@ def py_find_usages_result(path: str, name: str) -> Result[str]:
      if file.endswith(('.py', '.md', '.toml', '.txt', '.json')):
       _search_file(Path(root) / file)
   if not results:
+   if file_errors:
+    return Result(data=f"No usages found for '{name}' in {p}", errors=file_errors)
    return Result(data=f"No usages found for '{name}' in {p}")
   if len(results) > 100:
-   return Result(data="\n".join(results[:100]) + f"\n... (and {len(results)-100} more)")
-  return Result(data="\n".join(results))
+   return Result(data="\n".join(results[:100]) + f"\n... (and {len(results)-100} more)", errors=file_errors)
+  return Result(data="\n".join(results), errors=file_errors)
  except Exception as e:
   return Result(data="", errors=[ErrorInfo(kind=ErrorKind.INTERNAL, message=str(e), source="mcp.py_find_usages_result", original=e)])
 
@@ -935,6 +938,7 @@ def derive_code_path_result(target: str, max_depth: int = 5) -> Result[str]:
  from src.file_cache import ASTParser
  parser = ASTParser("python")
  found_path, found_code = None, None
+ file_errors: list[ErrorInfo] = []
  parts = target.split(".")
  symbol_name = parts[-1]
  if len(parts) > 1:
@@ -951,7 +955,9 @@ def derive_code_path_result(target: str, max_depth: int = 5) -> Result[str]:
       if _get_symbol_node(tree, symbol_name):
        found_path, found_code = str(p), code
        break
-     except Exception: continue
+     except (SyntaxError, ValueError) as e:
+      file_errors.append(ErrorInfo(kind=ErrorKind.INVALID_INPUT, message=f"failed to parse {p}: {e}", source="mcp.derive_code_path_result", original=e))
+      continue
    if found_path: break
  if not found_path:
   return Result(data="", errors=[ErrorInfo(kind=ErrorKind.NOT_FOUND, message=f"could not find definition for '{target}'", source="mcp.derive_code_path_result")])
@@ -987,13 +993,15 @@ def derive_code_path_result(target: str, max_depth: int = 5) -> Result[str]:
         break
       if c_path: break
     if c_path: trace(call, c_path, c_code, depth + 1, indent + "  ")
-  except Exception as e:
+  except (SyntaxError, ValueError, AttributeError) as e:
    output.append(f"{indent}  [!] Error parsing calls for {name}: {e}")
+   file_errors.append(ErrorInfo(kind=ErrorKind.INTERNAL, message=f"trace error for {name}: {e}", source="mcp.derive_code_path_result._trace", original=e))
  try:
   trace(symbol_name, found_path, found_code, 0, "")
-  return Result(data="\n".join(output))
+  return Result(data="\n".join(output), errors=file_errors)
  except Exception as e:
-  return Result(data="", errors=[ErrorInfo(kind=ErrorKind.INTERNAL, message=str(e), source="mcp.derive_code_path_result", original=e)])
+  file_errors.append(ErrorInfo(kind=ErrorKind.INTERNAL, message=str(e), source="mcp.derive_code_path_result", original=e))
+  return Result(data="", errors=file_errors)
 
 def get_tree_result(path: str, max_depth: int = 2) -> Result[str]:
  resolved = _resolve_and_check_result(path)