test(web): cover the error and response builders, and stop dropping empty values

errors.py and error_handler.py's response builders had no direct tests,
though every API response passes through them. Two bugs surfaced.

WebInterfaceError set suggested_fixes with `or`, so a caller passing []
to mean "I have no suggestions for this one" got the default list
instead. Only None should fall back.

create_success_response gated `data` on `is not None` but `message` and
`metadata` on truthiness, so an explicitly-passed "" or {} vanished from
the response while 0 and False survived — the response shape depended on
the value. api_helpers.success_response() then re-gated metadata the same
way, which is the path every api_v3 endpoint actually calls, so fixing
only the inner function would have changed nothing observable. Both now
use `is not None`.

That wrapper also merged request timing into the caller's own metadata
dict in place. A caller reusing a dict across requests would accumulate
previous responses' timings; it now copies before adding.

79 tests: category inference for every error code, mapped vs fallback
suggestions, the JSON shape including which keys are omitted when empty,
exception-to-code inference, and the success/error builders end to end.
Two behaviours are pinned as deliberate rather than fixed: an empty
context stays out of the response body, and from_exception's `message`
is the fixed per-code string, never the raw exception text.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NohXi78cwsAKtN1sCfxjUh

src/web_interface/error_handler.py

@@ -142,14 +142,17 @@ def create_success_response(
         "status": "success"
     }
     
+    # All three use `is not None` rather than truthiness: "" and {} are
+    # values a caller chose to send, and dropping them silently would make
+    # the response shape depend on the data.
     if data is not None:
         response["data"] = data
-    
-    if message:
+
+    if message is not None:
         response["message"] = message
-    
-    if metadata:
+
+    if metadata is not None:
         response["metadata"] = metadata
-    
+
     return response