Make multi-agent v2 ignore agents.max_depth (#20180)

## Why

`agents.max_depth` is a legacy multi-agent v1 guard. Multi-agent v2 uses
task-path routing and its own session/thread limits, so v2 should not
reject nested `spawn_agent` calls just because the thread-spawn depth
has reached the v1 maximum.

Keeping the v1 depth guard active in v2 prevents deeper task trees even
though the v2 path still needs the depth value only for lineage and
task-path metadata.

## What Changed

- Removed the depth-limit rejection from the multi-agent v2
`spawn_agent` handler while still computing child depth for lineage/path
metadata.
- Made the depth-based disabling of legacy `SpawnCsv`/`Collab` tools
apply only when `Feature::MultiAgentV2` is disabled.
- Added `multi_agent_v2_spawn_agent_ignores_configured_max_depth` to
cover a v2 child spawning another agent when `agent_max_depth = 1`,
while the existing v1 depth-limit tests continue to enforce the legacy
behavior.

## Verification

- `cargo test -p codex-core
multi_agent_v2_spawn_agent_ignores_configured_max_depth -- --nocapture`
- `cargo test -p codex-core depth_limit -- --nocapture`
- `cargo test -p codex-core tools::handlers::multi_agents::tests --
--nocapture`

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

@@ -489,6 +489,7 @@ impl Codex {
 
         if let SessionSource::SubAgent(SubAgentSource::ThreadSpawn { depth, .. }) = session_source
             && depth >= config.agent_max_depth
+            && !config.features.enabled(Feature::MultiAgentV2)
         {
             let _ = config.features.disable(Feature::SpawnCsv);
             let _ = config.features.disable(Feature::Collab);