Add startup configuration validation for prebuilt agents

[CommitHu502Craft]

Summary

• add fail-fast config validation when creating prebuilt agents
• require either LLM_MODEL or an explicit model=... value
• require JENTIC_AGENT_API_KEY for prebuilt agents and AWS_BEARER_TOKEN_BEDROCK for Bedrock variants
• add focused tests for missing-config and success paths

Closes #83.

Validation

• uv run pytest tests/agents/test_prebuilt.py -q
• uv run ruff check agents/prebuilt.py tests/agents/test_prebuilt.py
• uv run mypy --follow-imports=skip agents/prebuilt.py tests/agents/test_prebuilt.py
• $env:LLM_MODEL=''test-model''; uv run pytest -q

Notes

• I also installed the optional observability dependencies and tzdata locally so the full test suite could run cleanly on this Windows environment.
• No repository files related to dependency locking were included in the patch.