feat: validate DeepSeek file upload constraints (#19405)

## Background

DeepSeek consumers need invalid image uploads rejected locally to avoid
unnecessary requests and receive actionable errors.

## Summary

Added preflight validation for supported image formats, the 64 MiB size
limit, and 512-character filenames, with JPEG aliases and filename-based
fallback while preserving valid multipart requests and public types.

## Testing

Added Node and Edge coverage for JPEG, PNG, GIF, WebP, base64 input,
aliases, filename inference, exact boundaries, actionable
InvalidArgumentError failures, unchanged multipart data, and zero fetch
calls after rejection.

## End-to-end Validation

- Added and ran a focused DeepSeek upload example that demonstrated
local rejection and successfully uploaded a valid PNG using the live
provider API.

## Documentation

Updated the DeepSeek provider documentation with supported formats, size
and filename limits, alias handling, inference behavior, and content
validation precedence.

## Related Issues

Fixes #19389

Co-authored-by: ai-sdk-factory <308175966+ai-sdk-factory@users.noreply.github.com>
Co-authored-by: gr2m <39992+gr2m@users.noreply.github.com>