fix(context): reject app shells for binary downloads

[madtank]

Summary

• validate non-text context downloads against the actual HTTP response before writing bytes to disk
• reject HTML, JSON, and other suspicious text responses when a binary artifact was requested
• surface a clear error that includes the returned content type and a short response preview
• add a regression test covering the app-shell case

Validation

• pytest tests/test_context_commands.py -q
• live smoke against a known broken prod upload key now raises a clear ValueError instead of writing an HTML page as image.png

Context

When a gateway or upload URL resolves to an app shell or error page, axctl context download should fail loudly instead of silently saving garbage bytes. This is a guardrail in the CLI; it does not replace the backend/platform fix for missing upload blobs.

[madtank]

Focused validation on the current branch:

• python3 -m py_compile ax_cli/commands/context.py tests/test_context_commands.py
• pytest -q tests/test_context_commands.py
• python3 -m build
• result: 9 passed; sdist + wheel built successfully

QA packet to run before merge:

1. Run axctl context download against a known broken upload key that currently resolves to HTML/JSON.
2. Confirm the CLI errors clearly and does not write garbage bytes to disk.
3. Confirm a normal binary download still succeeds unchanged.