refactor: extract provider logic into modular registry

Each AI provider now has its own module implementing a ProviderModule
interface with createGateway, getAuthHeaders, and fetchModels methods.
This replaces the monolithic switch statements in ai-provider.ts and
models.post.ts with a clean registry pattern.

server/utils/providers/cohere.ts

@@ -0,0 +1,80 @@
+import { createCohere } from '@ai-sdk/cohere';
+import { GatewayFetchError } from '~~/server/utils/ai-provider';
+import { Err, Ok } from '~~/types/result';
+import type { ProviderModule } from '.';
+import { getModelData, mergeSets } from './helpers';
+
+export default {
+    id: 'cohere',
+    baseUrl: 'https://api.cohere.ai/v2',
+    modelsEndpoint: '/models',
+
+    createGateway({ apiKey, baseURL }) {
+        if (apiKey === undefined) {
+            return Err(GatewayFetchError.NoProviderApiKey);
+        }
+
+        return Ok({
+            gateway: createCohere({ apiKey, baseURL }),
+            streamTransformer: undefined,
+            textTransformer: undefined,
+        });
+    },
+
+    getAuthHeaders(apiKey) {
+        return apiKey ? { Authorization: `Bearer ${apiKey}` } : {};
+    },
+
+    async fetchModels({ baseURL, apiKey, modelsDevData }) {
+        const res = await fetch(`${baseURL}/models`, {
+            method: 'GET',
+            headers: { Authorization: `Bearer ${apiKey}` },
+        });
+        const data = await res.json();
+
+        if (!res.ok) {
+            throw createError({ statusCode: res.status, message: JSON.stringify(data) });
+        }
+
+        const models = [];
+        for (const model of data.models) {
+            const inputModalities = new Set<string>();
+            const capabilities = new Set<string>();
+
+            for (const feature of model.features || []) {
+                switch (feature) {
+                    case 'tools':
+                        capabilities.add('tools');
+                        break;
+                    case 'vision':
+                        inputModalities.add('image');
+                        break;
+                    case 'reasoning':
+                        capabilities.add('reasoning');
+                        break;
+                }
+            }
+
+            const modelData = getModelData(model.name, 'cohere', modelsDevData);
+
+            models.push({
+                ...modelData,
+                id: model.name,
+                attributes: {
+                    ...modelData.attributes,
+                    inputModalities: mergeSets(
+                        inputModalities,
+                        new Set(modelData.attributes?.inputModalities || [])
+                    ),
+                    capabilities: mergeSets(
+                        capabilities,
+                        new Set(modelData.attributes?.capabilities || [])
+                    ),
+                    contextWindow: model.context_length || modelData.attributes?.contextWindow,
+                }
+            });
+        }
+
+        return models;
+    },
+} satisfies ProviderModule;