fix(ui): block browser forwarding of CLI OAuth callback states

[mosure]

Motivation

• Prevent the browser-side OAuth callback handoff from forwarding attacker-controlled CLI OAuth state to the edge, which could enable session token exfiltration via malicious redirect URIs.
• Ensure browser forwarding only occurs for trusted feed:-encoded OAuth state that does not indicate a CLI client.

Description

• Tightened forwardRawGithubOauthCallback in crates/agent_feed_ui/src/reel.ts to decode the state value and refuse forwarding when it is not a decodable feed: payload or when the decoded client ends with -cli.
• Added decodeFeedOauthStateClient helper to safely parse the feed: hex-encoded JSON state and extract the client field, failing closed on malformed values.
• Updated UI snapshot/assertion expectations in crates/agent_feed_ui/src/lib.rs to reflect the new guard logic and helper function.

Testing

• Ran the UI unit test suite with cargo test -p agent_feed_ui --ignore-rust-version, iterating until expectations matched the updated output.
• Final automated test run succeeded: all UI tests passed (29 passed; 0 failed).

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Codex Task