getHarnessProfile

Look up the HarnessProfile for a model spec string.

Resolution order:

1. Exact match on spec (e.g., "openai:gpt-5.4").
2. Provider prefix (everything before :) when spec contains a colon and both halves are non-empty.
3. When both exist, they are merged (provider as base, exact as override).
4. undefined when nothing matches.

Malformed specs (empty, multiple colons, empty halves) return undefined without consulting the registry.