bug: `pytest -m "not ollama"` runs some tests that require ollama

[psschwei]

Was running uv run pytest -m 'not ollama and not qualitative' on an environment without ollama and have several tests fail as a result.

I believe these were the tests (with link to where it calls ollama):

• test/backends/test_tool_calls.py (

  mellea/test/backends/test_tool_calls.py

  Line 19 in 9cafe05

  return MelleaSession(backend=OllamaModelBackend(), ctx=ChatContext())

  )
• test/core/test_model_output_thunk.py (

  mellea/test/core/test_model_output_thunk.py

  Line 14 in 9cafe05

  m = start_session(model_options={ModelOption.MAX_NEW_TOKENS: 5})

  -- no backend == ollama)
• test/stdlib/test_functional.py (

  mellea/test/stdlib/test_functional.py

  Line 13 in 9cafe05

  m = start_session(model_options={ModelOption.MAX_NEW_TOKENS: 5})

  -- no backend == ollama)
• test/stdlib/requirements/test_requirement.py (

  mellea/test/stdlib/requirements/test_requirement.py

  Line 13 in 9cafe05

  m = start_session(ctx=ctx)

  -- no backend == ollama, all the tests in this file create their own session so there are multiple instances)

[psschwei]

This was fixed in #420