fix(api): showcase agent step runs on local Ollama (no API key needed)

[w7-mgfcode]

Root cause

On a local-Ollama stack (agent_default_model = ollama:qwen3:8b), the showcase agent_hitl_flow step (and legacy agent step) always ⏭️ skip with "no API key matching agent_default_model provider" — because _llm_key_present() in app/features/demo/pipeline.py only recognized cloud providers and returned False for ollama, which needs no API key.

Fix

Add an ollama branch to _llm_key_present() that returns True (no key required). The step still degrades gracefully if Ollama is unreachable — the chat round-trip fails and the step skips via its existing error path — so we let it attempt the call rather than pre-skip.

Tests

• test_llm_key_present_ollama_needs_no_key — ollama:qwen3:8b with no keys set → True.
• test_llm_key_present_cloud_still_requires_key — openai:* with empty key → False (cloud branch unchanged).

Scope note

Demo-only. Interactive Agent Chat (Chat page) builds the model via agents/base.py:build_agent_model (supports ollama:) and does not gate on _llm_key_present, so HITL already worked there on Ollama — this PR brings the showcase pipeline in line.

Validation

• ruff check ✅ · ruff format --check ✅
• mypy app/ ✅ — only pre-existing xgboost optional-dep import errors in untouched files
• pyright app/features/demo ✅ 0 errors
• pytest -m "not integration" ✅ 1653 passed, 12 skipped

Closes #340

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