fix: reject malformed scaled steering entries with 4xx

[RhizoNymph]

Summary

Malformed dict-style steering layer entries (e.g. {"vector": [...]} without "scale", or {"scale": 2.0} without "vector") caused a KeyError that bubbled up as a 500 Internal Server Error. This PR fixes the issue in two places:

• vllm/config/steering_types.py: normalize_layer_entry() now validates that both "vector" and "scale" keys are present in dict entries before accessing them. Missing keys raise a ValueError with a clear message listing exactly which keys are absent.

• vllm/entrypoints/serve/steering/api_router.py: The three _normalize_spec() calls that happened before the main try block are now wrapped in their own try/except catching KeyError, TypeError, and ValueError, returning a 400 Bad Request with a descriptive error message.

Four new unit tests in tests/v1/test_steering_types.py cover the missing-key scenarios (missing "scale", missing "vector", missing both, and verifying the error message content).

What changed and why

The root cause was that normalize_layer_entry blindly did entry["vector"] and entry["scale"] on dict entries without checking the keys existed first. Additionally, the normalization step in the API router happened outside any error handling, so even if the function raised a proper error it would still surface as a 500.

Both issues are fixed: the function now validates up front and raises ValueError (not KeyError), and the router catches normalization errors and returns 400.