fix: Add GPT-6 Astra to DeepNews model typing

[emmy-devop]

What

• Add gpt-6-astra to the direct DeepNewsModel type used by sync and async get_deep_news().
• Add regression coverage that keeps the model out of general chat, forecasts, and all Alerts model contracts.

Why

Align the hand-written SDK's direct DeepNews typing with the approved model rollout without changing request serialization, defaults, documentation, or Alerts behavior.

How I tested

• ruff check asknews_sdk (the command underlying task lint; task was unavailable in the execution environment)
• coverage run -m pytest --junitxml=report.xml, plus coverage report/XML/HTML generation (the commands underlying task test; 94 passed)
• git diff --check
• gitlint --commits HEAD~1..HEAD

Risk / rollback

Low: this is a typing-only catalog addition plus regression tests. No DTO, default, README, CHANGELOG, package-version, Alerts, or workflow files changed. This branch and PR trigger CI checks only; no release action is performed. A human merge to main would invoke the SDK's automated patch-release flow because the commit is fix:. Revert the commit to roll back.