cli: add strict config to exec-server

[bolinfest]

Why

PR #20559 added opt-in strict config parsing to the config-loading command surfaces, but codex exec-server was left out. That meant codex exec-server --strict-config was rejected even though the command can load config for remote registration, and local server startup had no way to fail fast on misspelled config keys.

What Changed

• Added --strict-config to codex exec-server.
• Allowed root-level inheritance from codex --strict-config exec-server.
• Validated config before local exec-server startup when strict mode is requested.
• Reused the loaded strict-config-aware config for remote exec-server registration auth.
• Added CLI coverage showing codex exec-server --strict-config rejects unknown config fields.

Verification

• cargo test -p codex-cli
• New integration test: strict_config_rejects_unknown_config_fields_for_exec_server

Documentation

Any strict-config command list on developers.openai.com/codex should include codex exec-server with the other supported config-loading entry points.

[bolinfest]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🚀

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