feat: provider modules — ProviderProfile ABC, 29 providers, fetch_models, transport single-path
Introduces providers/ as the single source of truth for every inference
provider. All 29 providers declared with correct data cross-checked against
auth.py, runtime_provider.py and auxiliary_client.py.
Rebased onto main (30307a980). Incorporates post-salvage fixes from
56724147e (gmi aux model google/gemini-3.1-flash-lite-preview, already set in providers/gmi.py).
