fix(proxy-responses): tolerate structured content type values

[Komzpa]

Summary

• guard explicit prompt-cache content-type checks so structured type values do not raise TypeError
• keep prompt-cache breakpoint stripping limited to known string content types
• add regression coverage through the API-key usage estimation path

Context

This is a follow-up guard for #1565. That PR strips subscription-incompatible explicit prompt-cache controls, but the recursive walker assumed every type value was hashable. Some Codex client payloads can carry structured content metadata, so the membership check must first confirm that type is a string.

Test

• uv run pytest -q tests/unit/test_proxy_api_key_usage.py

[chatgpt-codex-connector]

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