fix: stabilize /browser and /diff

[zemaj]

Summary

• /browser or /chrome not working #508: keep CDP browser sessions alive by treating chromiumoxide oneshot cancellations as non-fatal, and ignore proxy env vars during localhost CDP /json/version discovery.
• /diff is having off by one space err #473: strip the context prefix space when parsing diffs so /diff context lines align correctly.
• Add regression coverage for proxy env vs CDP discovery.

Tests

• cargo test -p code-browser --manifest-path code-rs/Cargo.toml --lib
• cargo test -p code-tui --manifest-path code-rs/Cargo.toml --lib history_cell::diff::tests::diff_record_strips_context_prefix_space
• ./build-fast.sh

Fixes #508
Fixes #473

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve proxies for non-local CDP discovery

Calling Client::builder().no_proxy() here disables all proxy use for discover_ws_via_host_port, not just localhost. If a user sets connect_host to a remote Chrome endpoint that is only reachable via HTTP(S)_PROXY (common in corporate networks), discovery will now bypass the proxy and fail to reach /json/version. The change is intended for localhost, but it regresses remote/proxied CDP setups by forcing direct connections.

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