fix: add input validation across model_hub, conversions and alpha

[crazywriter1]

Summary

• model_hub.py — Guard response.json() in upload() against HTML error pages: when the server returns a non-JSON response (e.g. a WAF or proxy error page), the previous code crashed with JSONDecodeError; now falls back to response.text. Note: create_model's successful-response response.json() call (HTTP 200 path) is intentionally out of scope — a 200 response returning HTML is not a realistic failure scenario.
• model_hub.py — Replace bare Exception with RuntimeError in create_model and create_version across all raise paths; fix create_version docstring which still claimed Raises: Exception; remove redundant except Exception: raise block that served no purpose
• _conversions.py — Add type guard at the top of convert_to_model_output: if event_data is None or not dict-like, raises TypeError with a clear message instead of an unhelpful AttributeError mid-function
• alpha.py — Validate inference_contract_address in the Alpha constructor using Web3.is_address() so invalid addresses fail immediately at construction time with a clear ValueError instead of failing silently during a transaction

Files Changed

• src/opengradient/client/model_hub.py
• src/opengradient/client/_conversions.py
• src/opengradient/client/alpha.py
• tests/client_test.py

Test Plan

• Full test suite passes: uv run pytest tests/ -v → 122 passed
• TestModelHubUploadErrorHandling — verifies HTML response raises RuntimeError, JSON error response raises RuntimeError with detail message
• TestConvertToModelOutputGuard — verifies None and non-dict inputs raise TypeError, empty dict returns empty output
• TestAlphaAddressValidation — verifies invalid address raises ValueError at construction time, valid address passes, empty string raises ValueError; Web3.is_address is called before address assignment (constructor-level validation, not checksum rule testing)

[crazywriter1]

Can you check this? @adambalogh