Show OAuth error descriptions in callback responses

[blevy-oai]

Motivation

• The local OAuth callback server returned a generic "Invalid OAuth callback" on failures even when the query contained an error_description, making it hard to debug OAuth failures.

Description

• Update codex-rs/rmcp-client/src/perform_oauth_login.rs to surface error_description values from the callback query in the HTTP response.
• Introduce a CallbackOutcome enum and change parse_oauth_callback to return it, parsing code, state, and error_description from the query string.
• Change spawn_callback_server to match on CallbackOutcome and return OAuth error: <description> with a 400 status when error_description is present, while preserving the existing success and invalid flows.
• Use inline formatting for the error response string.

Testing

• Ran just fmt in the codex-rs workspace to format changes successfully.
• Ran cargo test -p codex-rmcp-client and all tests passed.

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Codex Task

[blevy-oai]

Flows that fail with an error_description in the query now have a nice informative error in the body which is displayed in the browser. This makes debugging failures easier as now you don't have to copy the path out of the url bar and inspect it in a text editor