feat: enable Streamable HTTP MCP servers & lazy loading of tools

[seratch]

This pull request adds the following features:

1. Add Streamable HTTP MCP server support
2. Lazy loading of MCP server tools without blocking the shell initialization (resolves Optional / Lazy Loading of MCP Servers #2335)

If we want only 2. (lazy loading), I can send a separate pull request for it.

[github-actions]

Summary
Adds Streamable HTTP MCP server support and lazily loads MCP tools to avoid blocking shell startup. Updates config schema/docs, core wiring, mcp-client transport, and TUI status rendering.

Notes

• PR body is clear; consider briefly stating the motivation (startup latency + remote MCP support) and a short before/after UX note, plus a screenshot of the /mcp view.

Review
Thoughtful design and clean integration; a few nits and polish items to consider:

• codex-rs/mcp-client/src/mcp_client.rs
  • Prefer anyhow’s formatting over pre-formatting: replace anyhow!(format!(...)) with anyhow!(\"... {} ...\", x, y).
  • In send_request_http, content-type checks look good; consider handling charset (e.g., starts_with(\"application/json\") is fine) and returning the raw status on non-JSON/SSE responses for better diagnostics.
  • Minor fmt nit: format!(\"{} {}\", command, args.join(\" \")) can be format!(\"{command} {}\", args.join(\" \")).
• codex-rs/core/src/mcp_connection_manager.rs
  • Nice lazy init with OnceCell; good timeouts. Consider bubbling per-server initialize errors into the errors map (similar to spawn failures) when refresh_tools() is called, so a /mcp refresh can surface them explicitly.
  • qualify_tools: truncation + SHA1 suffix is solid. Since we allow only ASCII in names, slicing is safe; add a short comment noting the ASCII precondition (already implied by validation).
• codex-rs/core/src/codex.rs
  • Great that tool loading is kicked off post SessionConfigured. Consider emitting a brief “loading MCP tools…” event when any MCP servers are configured, so UIs can indicate initial loading state deterministically.
• codex-rs/core/src/config_types.rs
  • Streamable HTTP variant looks good. Consider documenting default header expectations (e.g., auth header) next to the type, matching the docs.
• codex-rs/tui/src/history_cell.rs
  • Rendering looks good; for style consistency, prefer ratatui’s Stylize helpers over Span::styled(…, Style::default()…) where possible (see styles.md).
  • New /mcp output paths are user-facing; please update/accept snapshot tests accordingly.
• codex-rs/mcp-client/Cargo.toml
  • Keep [dependencies] alphabetized per repo conventions.

Testing and snapshots

• Please run and accept TUI snapshots if expected: cargo test -p codex-tui, cargo insta pending-snapshots -p codex-tui, then cargo insta accept -p codex-tui if all good.
• Rust fmt/lints: from codex-rs/, run just fmt and just fix -p codex-mcp-client (and codex-core if needed).

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