test(core): fix late-suite harness regressions

[dzianisv]

Summary

• keep the default injected core test model catalog additive instead of treating it as an authoritative custom catalog
• add regression tests that distinguish the default test catalog path from explicit custom model catalogs
• make stdio_server_bin() build codex-rmcp-client's test_stdio_server on demand under Cargo so fresh worktrees can run the RMCP-backed core suites

Root Cause

• TestCodexBuilder was routing every config with a model_catalog through ThreadManager::new(...), which made the default injected test catalog authoritative and disabled remote /models refresh.
• core integration tests that depend on test_stdio_server assumed the cross-package binary was already built, which is false in a fresh worktree running only codex-core tests.

Testing

• cargo test -p core_test_support --lib -- --nocapture
• cargo test -p codex-core --test all suite::remote_models::remote_models_truncation_policy_with_tool_output_override -- --nocapture --exact
• cargo test -p codex-core --test all suite::search_tool::search_tool_selection_persists_across_turns -- --nocapture --exact
• cargo test -p codex-core --test all suite::truncation::mcp_tool_call_output_exceeds_limit_truncated_for_model -- --nocapture --exact
• cargo test -p codex-core --test all suite::rmcp_client::stdio_server_round_trip -- --nocapture --exact
• cargo test -p codex-core --test all suite::sqlite_state::mcp_call_marks_thread_memory_mode_polluted_when_configured -- --nocapture --exact
• cargo test -p codex-core --test all suite::search_tool:: -- --nocapture
• cargo test -p codex-core --test all suite::rmcp_client:: -- --nocapture

Fixes #26

[dzianisv]

CI status after opening this PR:

• The issue-#26 code changes are in and the local validation set passed.
• PR #30 is currently blocked by the separate workflow issues already tracked as #27 and #28.

What the failing checks are showing right now:

• build-test is failing inside scripts/stage_npm_packages.py because it cannot resolve a rust-release workflow for CODEX_VERSION=0.74.0.
• The rust-ci matrix jobs are still failing before any steps run. The run metadata again shows empty steps arrays for those matrix entries.
• The same runner/bootstrap pattern is also hitting the macOS Bazel matrix jobs.

I updated issue #26 with the actual two root causes fixed in this PR and the exact local validation set:

• https://github.com/dzianisv/codex/issues/26#issuecomment-4072206406

Next follow-on work is still #27 / #28 so this PR can turn green and merge cleanly.

[dzianisv]

Superseded by #35, which now carries this branch's changes on the consolidated integration branch.