[codex] fix CreateThreadParams test initializer

[anp-oai]

Summary

• initialize selected_capability_roots in the new attach_in_memory_thread_store test helper
• restore codex-core test compilation on main

Root cause

#30144 added the helper from commit 0c3d0742, whose parent was c38b2e9b. That branch was based before #29856 added selected_capability_roots as a required field on CreateThreadParams.

The PR's Rust and Bazel workflows both passed against the stale branch head 0c3d0742. When #30144 was squashed onto newer main, its initializer was integrated alongside the required field from #29856, producing E0063 in core/src/session/tests.rs. Because those workflows tested the branch head rather than the integrated merge result, they did not see the version-skew failure before merge.

Impact

Any job that compiles the codex-core library tests fails, which turned the main-branch rust-ci-full and Bazel workflows red across platforms and blocks unrelated focused core tests. This change only completes the test initializer; it does not alter production behavior or workflow configuration.

Validation

• just fmt
• just test -p codex-core turn_complete_flushes_terminal_event_after_delivery (1 passed, 2909 skipped)
• git diff --check

[teddywyly-oai]

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