fix(feedback): warn when feedback history files are unreadable

[694410194]

Summary

• load feedback history entries through a helper that keeps valid entries while collecting read/parse warnings for corrupted JSON files
• print those warnings to stderr instead of silently dropping bad files
• stop claiming No feedback history found. in text mode when the directory contains unreadable feedback entries, and add focused helper tests

Verification

• git diff --check
• static check that run_history now calls load_feedback_entries(&feedback_dir) and prints warnings to stderr
• added focused tests covering invalid JSON warnings and mixed valid+invalid entry loading
• rustc --version (not available in this environment, so cargo-based checks could not be run here)

Fixes #53182
/claim #53182

[greptile-apps]

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