fix: refine the warning message and docs for deprecated tools config (#7685)

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]`

codex-rs/core/tests/suite/deprecation_notice.rs

@@ -36,7 +36,7 @@ async fn emits_deprecation_notice_for_legacy_feature_flag() -> anyhow::Result<()
     let DeprecationNoticeEvent { summary, details } = notice;
     assert_eq!(
         summary,
-        "`use_experimental_unified_exec_tool` is deprecated. Use `unified_exec` instead."
+        "`use_experimental_unified_exec_tool` is deprecated. Use `[features].unified_exec` instead."
             .to_string(),
     );
     assert_eq!(