Fix retired model test expectations

[cbusillo]

Summary

• replace retired 5.2/5.3 Codex test fixtures with supported 5.5/5.4 fixtures
• update model picker expectations for the current GPT-5.5 and GPT-5.4 context behavior
• rename the unsupported 1M context test to cover GPT-5.5 instead of GPT-5.4

Validation

• cd code-rs && cargo test -p code-core context_mode_one_m_is_inert_for_unsupported_models -- --nocapture
• cd code-rs && cargo test -p code-tui model_selection_keeps_model_header_visible_for_first_entry -- --nocapture
• cd code-rs && cargo test -p code-tui model_selection_shows_available_context_hint_for_supported_model -- --nocapture
• cd code-rs && cargo test -p code-tui missing_builtins_default_to_disabled -- --nocapture
• cd code-rs && cargo test -p code-tui custom_agents_are_counted -- --nocapture
• cd code-rs && cargo test -p code-tui model_command_accepts_supported_non_curated_remote_model -- --nocapture
• ./build-fast.sh