[enhancement]: validate Azure's CustomData is base64-decodable

# Enhancement

## Summary
Validate that the `CustomData` field is base64-decodable while parsing `ovf-env.xml` on the Azure datasource, and raise a dedicated ReportableError when it cannot be decoded instead of letting provisioning fail in a less visible way.

## Motivation
Azure delivers user-provided `customData` to the guest as a base64-encoded value inside the provisioning OVF (`ovf-env.xml`). Today, cloud-init decodes this field with `base64.b64decode(...)` during OVF parsing. If the value is not valid base64, `b64decode` raises a bare `binascii.Error` that is not caught at the parsing boundary, so cloud-init proceeds and the failure surfaces later as an unhandled exception which is a bit less clear to diagnose and does not get reported to the platform through the normal KVP path.

Locate the Azure datasource code that parses CustomData from ovf-env.xml and the existing ReportableError handling boundary. Start by tracing the base64.b64decode call and inspect nearby tests or parsing entry points. Done means invalid CustomData raises a dedicated ReportableError while valid values continue through parsing and the error is reported through the normal KVP path.

Written by the indexing model from the issue text.