fix(proxy): strip OpenRouter hop-by-hop request headers

[yovinchen]

Summary

• strip hop-by-hop request headers only for Codex requests routed to OpenRouter chat/responses endpoints
• keep Claude official messages requests unchanged and leave response-side behavior out of this patch
• add focused unit coverage for the OpenRouter guard and dynamic Connection token stripping

Testing

• cargo test --manifest-path src-tauri/Cargo.toml should_guard_openrouter_codex_headers_only_for_openrouter_chat_and_responses
• cargo test --manifest-path src-tauri/Cargo.toml should_strip_openrouter_codex_request_header_covers_static_and_dynamic_hop_by_hop_headers
• cargo test --manifest-path src-tauri/Cargo.toml rewrite_claude_transform_endpoint_strips_beta_for_chat_completions
• cargo clippy --manifest-path src-tauri/Cargo.toml --lib -- -W clippy::too_many_arguments

Closes #1888

[yovinchen]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Can't wait for the next one!

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[farion1231]

Hi, I owe you an apology for the long silence. As the sole maintainer of this project, I've been overwhelmed and fell behind on reviews — but that's on me, not on you. Your contribution matters and I will review this PR. If it needs a rebase, just let me know or I can handle it. Thank you for your time and patience.