🧪 Improve process-mappings consumption testing

[edithatogo]

🎯 What: The testing gap in validate_consumption error branches was addressed. Added extensive negative testing permutations that test schema digest mismatch, schema path resolution, empty inputs, non-dictionary components, missing keys, and invalid property structures that were previously uncovered.
📊 Coverage: Achieved 100% test coverage for src/pic_contracts/process_mappings_consumption.py, up from ~64%. All logical branches are covered, validating robust failure mode behavior.
✨ Result: Robust regression safety for local consumption tests on pinned external manifests. Ensures the library correctly rejects invalid schemas, bad formatting, escaped directory paths, and malformed contract parameters.

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PR created automatically by Jules for task 2914775939374426167 started by @edithatogo

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