feat: catalog multi-agent v2 config (#26254)

## Why

Model metadata can now select multi-agent v2 even when a user has not
enabled `features.multi_agent_v2` in their config. Some existing configs
still set the legacy `agents.max_threads` knob for v1 multi-agent
behavior, so treating every v2 runtime as incompatible with
`agents.max_threads` would break users whose only v2 signal came from
the model catalog.

The incompatible configuration is specifically enabling
`features.multi_agent_v2` while also setting `agents.max_threads`.
Catalog-forced v2 should use the v2 concurrency setting and ignore the
legacy v1 cap instead of rejecting the config.

## What changed

- Split config validation from runtime concurrency calculation:
`effective_agent_max_threads` now just returns the effective cap for the
resolved multi-agent runtime.
- Added explicit validation for `features.multi_agent_v2` +
`agents.max_threads` at session startup.
- Preserved catalog-selected v2 behavior when `features.multi_agent_v2`
is disabled, so existing configs with `agents.max_threads` keep
starting.
- Updated model-runtime selector coverage so a catalog v2 model still
exposes v2 tools even when `agents.max_threads` is set and the config
flag is disabled.

## Validation

- `cargo check -p codex-core --lib`
- `just test -p codex-core --lib -E
"test(multi_agent_v2_feature_rejects_agents_max_threads) |
test(catalog_v2_allows_agents_max_threads_when_feature_disabled)"`

codex-rs/core/src/config/mod.rs

@@ -1286,26 +1286,29 @@ impl Config {
         }
     }
 
+    pub(crate) fn validate_multi_agent_v2_config(&self) -> std::io::Result<()> {
+        if self.features.enabled(Feature::MultiAgentV2) && self.agent_max_threads.is_some() {
+            Err(std::io::Error::new(
+                std::io::ErrorKind::InvalidInput,
+                "agents.max_threads cannot be set when features.multi_agent_v2 is enabled",
+            ))
+        } else {
+            Ok(())
+        }
+    }
+
     pub(crate) fn effective_agent_max_threads(
         &self,
         multi_agent_version: MultiAgentVersion,
-    ) -> std::io::Result<Option<usize>> {
+    ) -> Option<usize> {
         match multi_agent_version {
-            MultiAgentVersion::V2 => {
-                if self.agent_max_threads.is_some() {
-                    return Err(std::io::Error::new(
-                        std::io::ErrorKind::InvalidInput,
-                        "agents.max_threads cannot be set when the multi-agent runtime is v2",
-                    ));
-                }
-                Ok(Some(
-                    self.multi_agent_v2
-                        .max_concurrent_threads_per_session
-                        .saturating_sub(1),
-                ))
-            }
+            MultiAgentVersion::V2 => Some(
+                self.multi_agent_v2
+                    .max_concurrent_threads_per_session
+                    .saturating_sub(1),
+            ),
             MultiAgentVersion::Disabled | MultiAgentVersion::V1 => {
-                Ok(self.agent_max_threads.or(DEFAULT_AGENT_MAX_THREADS))
+                self.agent_max_threads.or(DEFAULT_AGENT_MAX_THREADS)
             }
         }
     }