Validate Exec CLI in multitool wrapper and update fork/resume test

[friel-openai]

Motivation

• Ensure the multitool codex wrapper enforces the same ExecCli validation rules as the standalone codex-exec binary so codex exec --fork <id> resume is rejected consistently and the related unit test no longer fails.

Description

• Call ExecCli::validate() in the multitool dispatch path before forwarding to codex_exec::run_main and handle validation errors by exiting appropriately; change located in codex-rs/cli/src/main.rs.
• Update the unit test exec_fork_conflicts_with_resume_subcommand to parse the CLI and assert that exec.validate() returns an error (parse succeeds but validation fails) to reflect where the conflict is enforced.

Testing

• Ran formatting with just fmt in the workspace and it completed successfully.
• Executed the failing unit case with: PKG_CONFIG_PATH=/tmp/libcap-shim/lib/pkgconfig NO_PROXY=127.0.0.1,localhost cargo test -p codex-cli tests::exec_fork_conflicts_with_resume_subcommand, and the test passed.
• Ran the codex-exec and codex-cli test suites with the same environment shim (PKG_CONFIG_PATH=/tmp/libcap-shim/lib/pkgconfig NO_PROXY=127.0.0.1,localhost) and they completed successfully.

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Codex Task