[Feat] Provider backend registry for multi-backend support and better code maintenance

[PyDevC]

Files: lua/model_cmp/modelapi/common.lua:44-57

Feature Introduction: Currently adding a new AI backend requires modifying common.lua's if-else chain. A registry pattern lets any backend register itself with a single call.

Feature Description:

-- lua/model_cmp/modelapi/init.lua
function M.register(name, provider)
    providers[name] = provider
end

-- lua/model_cmp/modelapi/codestral.lua
require("model_cmp.modelapi").register("codestral", {
    generate_request = function(prompt) ... end,
})

Benefits:

• New backends are single-file additions
• User can configure which backends to load
• Clean separation of concerns

Is your feature possible with the current status of repo? Yes. Pure refactor — no behavior changes needed.