fix: show all chat models and handle auto-resolved model not in cache

[Copilot]

On accounts where the Copilot API returns model_picker_enabled: false for all models, the picker showed only "Auto (Copilot)", and selecting it crashed with Resolved model not found: <model-id>.

Changes

lua/CopilotChat/config/providers.lua — get_models

Previously filtered strictly to model_picker_enabled = true. Now uses a two-pass approach: prefer picker-enabled models, but fall back to all chat models when none are picker-enabled. This ensures real models appear in the picker for restricted accounts.

-- Before: empty list when model_picker_enabled is false for all models
:filter(function(model)
  return model.capabilities.type == 'chat' and model.model_picker_enabled
end)

-- After: fall back to all chat models
local all_chat_data = vim.tbl_filter(function(m) return m.capabilities and m.capabilities.type == 'chat' end, data)
local picker_data   = vim.tbl_filter(function(m) return m.model_picker_enabled end, all_chat_data)
vim.iter(#picker_data > 0 and picker_data or all_chat_data):map(...)

lua/CopilotChat/client.lua — Client:ask

When resolve_model returns a model id not present in the cache (because it was filtered out), previously errored hard. Now falls back to the originating model's config (e.g. the auto entry) so the request proceeds with the correct model id set in options.model.id.

-- Before: hard error
model_config = models[opts.model]
if not model_config then error('Resolved model not found: ' .. opts.model) end

-- After: graceful fallback to original config
model_config = models[opts.model] or model_config