feat: re-export serve types from public mellea.* namespace

[planetf1]

Problem

m serve user programs must type their serve() input parameter as list[ChatMessage], because that is what the framework passes in. The only way to import ChatMessage today is:

from cli.serve.models import ChatMessage

cli/ is not part of the installed mellea package. This import works when running from the repo root, but fails for anyone who has installed mellea as a library and is writing their serve script outside the repo:

ModuleNotFoundError: No module named 'cli'

All existing examples in docs/examples/m_serve/ use this pattern, and PR #823 adds another instance.

Expected behaviour

Serve types needed by user programs should be importable from a stable public path, e.g.:

from mellea.serve import ChatMessage

Scope

At minimum ChatMessage needs a public export path. It is worth auditing cli/serve/models.py for any other types that user-facing serve() functions are expected to use and including them in the same export.

The CLI internals can then import from mellea.serve rather than defining the types in cli/.

Affected files

• cli/serve/models.py — current home of ChatMessage
• docs/examples/m_serve/m_serve_example_simple.py
• docs/examples/m_serve/m_serve_example_streaming.py (added in feat: streaming support in m serve OpenAI API server #823)
• Any user code following the examples