improvement(providers): align attachment dispatch to vendor SDK types

Post-merge audit of #4610 surfaced three follow-ups:

1. xAI Grok vision was blocked. Grok runs through the OpenAI-compatible
   chat-completions endpoint, so removing xAI from UNSUPPORTED_FILE_PROVIDERS
   and routing it through the image-only branch restores image attachments
   on vision models.

2. Azure OpenAI chat-completions deployments blocked any file attachment.
   Added a per-message image_url parts path; documents still require the
   Responses API endpoint and throw a clear, actionable error.

3. Wire shapes were loosely typed (Record<string, unknown> arrays).
   Replaced with `satisfies` clauses against each vendor SDK union at every
   push site: OpenAI Responses/Chat, Anthropic ContentBlockParam, Gemini
   Part, Bedrock ContentBlock members. AnthropicImageMediaType now derives
   from Base64ImageSource['media_type'] so it tracks SDK updates.

Also collapsed the validation cascade into an exhaustive switch with
`never` enforcement, and dropped the redundant per-provider
formatMessagesForProvider call from xai/index.ts (providers/index.ts
already runs the dispatcher centrally).

Author: waleedlatif1

apps/sim/providers/attachments.test.ts

@@ -66,6 +66,7 @@ describe('provider attachments', () => {
       {
         type: 'input_image',
         image_url: 'data:image/png;base64,iVBORw0KGgo=',
+        detail: 'auto',
       },
       {
         type: 'input_file',