fix(model_switch): section 3 base_url/model/dedup follow-up

On top of the salvaged PR #12505 (Jason/farion1231, which adds dict-format
models: enumeration to both sections), three section-3 refinements from
competing PR #11534 (YangManBOBO):

- accept base_url as canonical (matches Hermes's writer and custom_providers
  entries); keep api/url as fallbacks for legacy/hand-edited configs
- accept singular model as a default_model synonym, matching custom_providers
- add seen_slugs guard so the same provider slug appearing in both
  providers: dict and custom_providers: list emits exactly one picker row
  (providers: dict wins since section 3 runs first)

Two regression tests cover the new behavior. AUTHOR_MAP entry added for
farion1231 so CI doesn't reject the cherry-picked commit.

scripts/release.py

@@ -177,6 +177,7 @@ AUTHOR_MAP = {
     "duerzy@gmail.com": "duerzy",
     "emozilla@nousresearch.com": "emozilla",
     "fancydirty@gmail.com": "fancydirty",
+    "farion1231@gmail.com": "farion1231",
     "floptopbot33@gmail.com": "flobo3",
     "fontana.pedro93@gmail.com": "pefontana",
     "francis.x.fitzpatrick@gmail.com": "fxfitz",