feat(network-proxy): structured policy signaling and attempt correlation to core (#11662)

## Summary
When network requests were blocked, downstream code often had to infer
ask vs deny from free-form response text. That was brittle and led to
incorrect approval behavior.
This PR fixes the proxy side so blocked decisions are structured and
request metadata survives reliably.

## Description
- Blocked proxy responses now carry consistent structured policy
decision data.
- Request attempt metadata is preserved across proxy env paths
(including ALL_PROXY flows).
- Header stripping was tightened so we still remove unsafe forwarding
headers, but keep metadata needed for policy handling.
- Block messages were clarified (for example, allowlist miss vs explicit
deny).
- Added unified violation log entries so policy failures can be
inspected in one place.
- Added/updated tests for these behaviors.

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Co-authored-by: Codex <199175422+chatgpt-codex-connector[bot]@users.noreply.github.com>

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