fix: small correctness/security cleanups in experimental skill scripts

[jamesbroadhead]

Summary

Batch of small, contained fixes surfaced by a parallel GPT 5.4 + Gemini 3.1 Pro code review. None of these are introduced by recent changes — they are pre-existing issues that ought to land regardless. Each fix is local and review-by-eye.

databricks-agent-bricks/scripts/mas_manager.py

• _delete now tolerates empty / non-JSON DELETE response bodies (was raising JSONDecodeError on successful deletes).
• add_examples_batch short-circuits on empty input (ThreadPoolExecutor rejects max_workers=0).
• _build_agent_list validates uc_function_name has 3 dotted parts (was IndexError on malformed) and rejects missing endpoint_name with a clear error (was sending {"name": null} to the API).
• main() uses a _parse_json_arg helper that exits cleanly on malformed JSON CLI args, replacing raw json.loads tracebacks.

databricks-execution-compute/scripts/compute.py

• manage-cluster --action get now keys off the SDK's typed NotFound exception instead of substring-matching "does not exist" in the message. Also flips success to false for the DELETED state so callers gating on success don't treat a missing cluster as a successful lookup.

databricks-app-python/examples/llm_config.py (security)

• OAuth error no longer interpolates the full token-endpoint payload (which can contain id_token / refresh material). Logs the present key names instead.
• DATABRICKS_MODEL validation error drops the response.text[:300] echo so server bodies don't end up in operator-visible error text.

databricks-app-python/examples/fm-minimal-chat.py

• Docstring + app.yaml examples reference the actual filename (fm-minimal-chat.py), not 2-minimal-chat-app.py.

databricks-app-python/examples/fm-parallel-calls.py

• Guard Speedup division on total_time > 0 to avoid ZeroDivisionError on fast paths.
• Convert the trailing standalone triple-quoted string (dead code) to real # comments.

databricks-python-sdk/examples/5-serving-and-vector-search.py

• Replace the [0.1, 0.2, 0.3, ...] literal-Ellipsis vector with a named placeholder + comment explaining it's a stand-in (the literal would fail server-side if copied).

Companion PR

Equivalent PR for main (smaller — mas_manager.py and compute.py don't exist under databricks-skills/ on main): https://github.com/databricks-solutions/ai-dev-kit/pull/new/ace-review-fixes-main (will be opened next)

Test plan

• python3 -m py_compile on all six modified files
• CI green
• Spot-check mas_manager.py delete_mas against a real workspace (DELETE response.json fix)

This pull request and its description were written by Isaac.

[jamesbroadhead]

Hi @calreynolds — could you take a look at this when you have a moment? Small set of fixes from a code review. Companion PR for main is #535.

— this comment was written by Claude