fix(waterdata)!: return (df, BaseMetadata) from get_codes (#301)

Every other public waterdata getter returns a (DataFrame, BaseMetadata)
tuple, but get_codes returned a bare DataFrame, so the module's universal
idiom `df, md = waterdata.get_codes(...)` raised an unpack error. It already
builds `response` (and calls _raise_for_non_200), so the metadata was in hand
and discarded.

BREAKING: callers doing `df = get_codes(...)` must switch to
`df, _ = get_codes(...)`.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: thodson-usgs

dataretrieval/waterdata/api.py

@@ -2101,7 +2101,7 @@ def get_reference_table(
     )
 
 
-def get_codes(code_service: CODE_SERVICES) -> pd.DataFrame:
+def get_codes(code_service: CODE_SERVICES) -> tuple[pd.DataFrame, BaseMetadata]:
     """Return codes from a Samples code service.
 
     Parameters
@@ -2110,6 +2110,13 @@ def get_codes(code_service: CODE_SERVICES) -> pd.DataFrame:
         One of the following options: "states", "counties", "countries"
         "sitetype", "samplemedia", "characteristicgroup", "characteristics",
         or "observedproperty"
+
+    Returns
+    -------
+    df : ``pandas.DataFrame``
+        The requested code table.
+    md : :obj:`dataretrieval.utils.BaseMetadata`
+        Metadata for the query (URL, query time, response headers).
     """
     valid_code_services = get_args(CODE_SERVICES)
     if code_service not in valid_code_services:
@@ -2129,7 +2136,7 @@ def get_codes(code_service: CODE_SERVICES) -> pd.DataFrame:
 
     df = pd.DataFrame(data_list)
 
-    return df
+    return df, BaseMetadata(response)
 
 
 def _get_samples_csv(