Fix structured startup errors for invalid env options

[erSpell]

Summary

• Validate EMBEDDER and LLM values when settings are loaded.
• Raise a structured StartupConfigError that records the invalid variable, value, and valid options.
• Report invalid CLI startup config as a clean configuration error instead of a traceback.
• Add regression coverage for invalid EMBEDDER, invalid LLM, and CLI no-traceback behavior.

Tests

• . .venv/Scripts/activate && python -m pytest tests/test_config.py -q
• . .venv/Scripts/activate && python -m pytest tests/test_config.py tests/test_embeddings.py -q
• . .venv/Scripts/activate && python -m pytest tests/test_config.py tests/test_embeddings.py tests/test_generate.py tests/test_response_models.py tests/test_cli_citations.py -q
• . .venv/Scripts/activate && python -m ruff check vaultrag/config.py vaultrag/cli.py tests/test_config.py && python -m compileall -q vaultrag

Notes

• The config-focused tests and nearby non-DB tests pass locally.

Closes #23

[github-actions]

Welcome, @erSpell, and thanks for your first pull request to VaultRAG.

A quick look at CONTRIBUTING.md covers how things are set up and run here.

I'll review this shortly. Ask anything in the thread if something is unclear.