Update Posta to 0.15.0 and expose template languages

[axies20]

Updates the Posta container image from 0.13.1 to 0.15.0 and exposes the languages array returned by the template list API through ListTemplatesResponseDataItem.Languages. The nullable property preserves compatibility with responses from older servers.

The hosting resource test now asserts the 0.15.0 tag. This is a follow-up to #1461; the existing integration is already in main.

PR Checklist

• Created a feature branch in a fork
• Based on the latest upstream main at preparation time
• No merge commits
• Existing container image assertion updated
• Contains no breaking API changes
• New property has XML documentation

Validation

• Posta client tests: 10 passed.
• Posta hosting tests: 22 passed, including container startup and health endpoint verification.
• git diff --check HEAD~1 HEAD passed.