fix(functions_client): use uppercase HTTP method in requests

[j8000]

Summary

• FunctionsClient.invoke() uses HttpMethod.name to set the HTTP method on requests, which returns lowercase ("post", "get", etc.)
• While IOClient and CupertinoClient normalize methods to uppercase internally, CronetClient (recommended Android HTTP client from cronet_http) sends the method verbatim
• The Supabase relay rejects lowercase methods with 400 Bad Request
• Fix: call .toUpperCase() on method.name for both http.Request and http.MultipartRequest

Fixes #1352

Test plan

• Call supabase.functions.invoke() with CronetClient as httpClient on Android — should no longer return 400
• Verify existing behavior unchanged with IOClient and CupertinoClient