fix: small correctness/security cleanups in apps-python / python-sdk examples

[jamesbroadhead]

Summary

Subset of fixes surfaced by a parallel GPT 5.4 + Gemini 3.1 Pro code review, scoped to files that exist on main. (The companion experimental-branch PR contains a larger superset; mas_manager.py and compute.py live only under databricks-skills/ on experimental.)

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 (larger) PR for experimental — adds fixes to mas_manager.py and compute.py which exist there: #534

Test plan

• python3 -m py_compile on all four modified files
• CI green

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? Subset of fixes for main. Companion PR for experimental (larger) is #534.

— this comment was written by Claude