fix(initializer): return actionable model STORAGE_URI errors

[sahilleth]

Summary

• Validate missing and empty STORAGE_URI with a descriptive ValueError instead of a raw KeyError
• Raise a stable ValueError for unsupported schemes, listing supported model providers (hf, s3)
• Add parameterized tests for missing, empty, Hugging Face, S3, and unsupported URI cases

Fixes #3834

Test plan

• pytest pkg/initializers/model/ -v (13 tests pass)
• Missing STORAGE_URI → ValueError("STORAGE_URI environment variable must be set")
• Empty STORAGE_URI → same ValueError
• invalid://model/path → ValueError with supported schemes
• hf:// and s3:// success paths unchanged

[google-oss-prow]

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