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/tests/suite/model_runtime_selectors.rs

@@ -190,6 +190,7 @@ async fn remote_multi_agent_selector_overrides_feature_flags() -> Result<()> {
     let mut v2_model = remote_model("test-multi-agent-v2");
     v2_model.multi_agent_version = Some(MultiAgentVersion::V2);
     let v2_body = response_body_for_remote_model(v2_model, |config| {
+        config.agent_max_threads = Some(3);
         config
             .features
             .enable(Feature::Collab)