fix(import): reject invalid whitespace-only or unknown message roles

[694410194]

Summary

• reject empty and whitespace-only export message roles before import mutates message content
• reject unknown roles instead of silently prefixing the imported text with bracketed role labels
• add focused unit coverage for empty, whitespace-only, and unknown roles

Verification

• git diff --check
• rustc --version (not available in this environment, so cargo-based tests could not be run here)

Fixes #53211
/claim #53211

[greptile-apps]

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