mtmd : harden feature_layer range check

[3em0]

Follow-up to #29276, which added a range check for clip.*.feature_layer (indices above n_layer are rejected). Two small hardenings:

• also reject negative indices: conversions are expected to standardize on non-negative values (-1 is used internally to mark values as unset), so a negative declared index is malformed and should fail loudly instead of being silently ignored
• include the offending key and the valid range in the error message, which makes a malformed projector easier to identify from the log

No behavior change for well-formed files.

Testing

• mmproj declaring feature_layer = [4096] with block_count = 2: fails to load with the range error, same as master
• mmproj declaring a negative feature_layer value: now fails to load with the range error (previously the value was silently ignored and the default layer choice was used)
• well-formed projectors with indices in [0, n_layer] (llava, minicpmv-family, granite-vision): load as before

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