fix(model_switch): group custom_providers by endpoint in /model picker (#9210)

Multiple custom_providers entries sharing the same base_url + api_key
are now grouped into a single picker row. A local Ollama host with
per-model display names ("Ollama — GLM 5.1", "Ollama — Qwen3-coder",
"Ollama — Kimi K2", "Ollama — MiniMax M2.7") previously produced four
near-duplicate picker rows that differed only by suffix; now it appears
as one "Ollama" row with four models.

Key changes:
- Grouping key changed from slug-by-name to (base_url, api_key). Names
  frequently differ per model while the endpoint stays the same.
- When the grouped endpoint matches current_base_url, the row's slug is
  set to current_provider so picker-driven switches route through the
  live credential pipeline (no re-resolution needed).
- Per-model suffix is stripped from the display name ("Ollama — X" →
  "Ollama") via em-dash / " - " separators.
- Two groups with different api_keys at the same base_url (or otherwise
  colliding on cleaned name) are disambiguated with a numeric suffix
  (custom:openai, custom:openai-2) so both stay visible.
- current_base_url parameter plumbed through both gateway call sites.

Existing #8216, #11499, #13509 regressions covered (dict/list shapes
of models:, section-3/section-4 dedup, normalized list-format entries).

Salvaged from @davidvv's PR #9210 — the underlying code had diverged
~1400 commits since that PR was opened, so this is a reconstruction of
the same approach on current main rather than a clean cherry-pick.
Authorship preserved via --author on this commit.

Closes #9210

gateway/run.py

@@ -5484,6 +5484,7 @@ class GatewayRunner:
                 try:
                     providers = list_authenticated_providers(
                         current_provider=current_provider,
+                        current_base_url=current_base_url,
                         user_providers=user_provs,
                         custom_providers=custom_provs,
                         max_models=50,
@@ -5595,6 +5596,7 @@ class GatewayRunner:
             try:
                 providers = list_authenticated_providers(
                     current_provider=current_provider,
+                    current_base_url=current_base_url,
                     user_providers=user_provs,
                     custom_providers=custom_provs,
                     max_models=5,