fix(picker): remove max_models=50 cap in interactive model pickers

The interactive model pickers (Desktop REST API, TUI model.options, CLI
/model) were hard-capped at max_models=50, which truncated large provider
catalogs like Kilo Gateway (336 models) to just 50 entries. This made
most models undiscoverable via the picker search box.

Changes:
- Change build_models_payload() default from max_models=50 to None (unlimited)
- Change list_authenticated_providers() default from max_models=8 to None
- Change list_picker_providers() default from max_models=8 to None
- Fix all [:max_models] slicing to handle None as 'no limit'
- Remove max_models=50 from 5 interactive picker callers:
  * web_server.py: get_model_options (Desktop /api/model/options)
  * web_server.py: get_recommended_default_model
  * model_switch.py: prewarm_picker_cache_async
  * tui_gateway/server.py: model.options JSON-RPC
  * cli.py: HermesCLI model picker
- Telegram/Discord inline keyboard picker (gateway/slash_commands.py)
  still passes max_models=50 explicitly — unchanged behavior.

The total_models field was already in the response payload and is now
meaningful since models.length == total_models for interactive pickers.

Fixes #48279

cli.py

@@ -6971,7 +6971,7 @@ class HermesCLI(CLIAgentSetupMixin, CLICommandsMixin):
             try:
                 if ctx is None:
                     raise RuntimeError("inventory context unavailable")
-                providers = build_models_payload(ctx, max_models=50)["providers"]
+                providers = build_models_payload(ctx)["providers"]
             except Exception:
                 providers = []