fix(proxy): bound usage scan buffers

[mstuart]

Summary

• enforce the documented 2 MiB head and 64 KiB tail response-scan limits per chunk
• avoid transiently buffering an arbitrarily large upstream chunk in the usage scanner
• add regression coverage for oversized chunks and rolling-tail behavior

Verification

• cargo fmt --check
• cargo clippy -p tare-proxy --all-targets -- -D warnings
• cargo test -p tare-proxy

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-25T20:10:30.059729Z	e08d28b	PR opened
🔒 Security Review	✅ Completed 2026-09-25T20:10:13.820663Z	e08d28b	PR opened

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