fix: refine the warning message and docs for deprecated tools config

[gameofby]

Issue #7661 revealed that users are confused by deprecation warnings like:

tools.web_search is deprecated. Use web_search_request instead.

This message misleadingly suggests renaming the config key from web_search to web_search_request, when the actual required change is to move and rename the configuration from the [tools] section to the [features] section.

This PR clarifies the warning messages and documentation to make it clear that deprecated [tools] configurations should be moved to [features]. Changes made:

• Updated deprecation warning format in codex-rs/core/src/codex.rs:520 to include [features]. prefix
• Updated corresponding test expectations in codex-rs/core/tests/suite/deprecation_notice.rs:39
• Improved documentation in docs/config.md to clarify upfront that [tools] options are deprecated in favor of [features]

[github-actions]

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[gameofby]

I have read the CLA Document and I hereby sign the CLA

[gameofby]

recheck

[etraut-openai]

Thanks for the contribution!