Enable parallel tool calls for Codex Apps

[soheil-oai]

Summary

• add a disabled-by-default codex_apps_parallel_tool_calls feature
• opt the host-owned codex_apps MCP server into parallel tool calls when enabled
• preserve existing behavior for user-configured and third-party MCP servers
• cover direct feature enablement, service-provided SessionThreadConfig.features, and end-to-end call scheduling

Why

Codex currently parallelizes MCP calls only when a server opts in with supports_parallel_tool_calls or a tool advertises readOnlyHint: true. First-party connectors are ready to accept parallel calls, but enabling the behavior globally would risk breaking third-party MCP servers that are not concurrency-safe.

This change provides a narrowly scoped rollout switch for the host-owned Codex Apps server. The backend Statsig evaluation and gradual rollout are tracked separately in CONNECT-811, under CONNECT-810.

Validation

• just test -p codex-features — 53 passed
• just test -p codex-core codex_apps_parallel_tool_calls — 2 passed
• just test -p codex-core codex_apps_calls_run_ — 2 passed (two clean runs)

Rebased onto origin/main at 0bbea86a6aae37b1f243676db4248000f04ad111 before the final push.

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[chatgpt-codex-connector]

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[pakrym-oai]

nit: do we have an existing app-related integration test file?

[soheil-oai]

sohex here (Soheil's Codex) - There are Apps-oriented suites such as search_tool.rs and mcp_tool_exposure.rs, but this test exercises MCP execution scheduling rather than search/exposure. search_tool.rs is already ~1.6k lines, so I kept the two parallelism cases in the focused 190-line suite rather than grow that large module.