fix(cli): gate test-only selector and model helpers behind cfg(test)

[1688mengdie]

Summary

Fixes #2567

Gate three CLI helpers that are referenced exclusively from #[cfg(test)] modules behind #[cfg(test)] (one attribute line each):

• resolve_session_model_display_id in src/apps/cli/src/model_selection.rs
• AgentSelectorState::show_modes_only in src/apps/cli/src/ui/agent_selector.rs
• ChatView::show_agent_modes_only in src/apps/cli/src/ui/chat/popups.rs

Production builds no longer carry the three dead symbols or emit their dead-code warnings; the unit tests keep using the helpers unchanged (see issue #2567 for the run evidence with file:line and rule names).

Type and Areas

Type: bug fix (build warnings cleanup)

Areas: Rust CLI

Motivation / Impact

No direct user-facing change. Non-test builds stop emitting 3 dead-code warnings on every full CI run of main, and the production binary stops carrying test-only symbols.

Verification

• cargo check --locked -p bitfun-cli on Windows: exit 0, the three dead_code warnings gone (crate baseline 19 warnings down to 16 on Windows; the remaining ones are other symbols covered by separate PRs).
• Focused tests still pass with the gates in place: cargo test --locked -p bitfun-cli --bin bitfun model_selection (6 passed), ... agent_selector (12 passed), ... pending_mode_update — the show_agent_modes_only consumer test (1 passed), 0 failed overall.
• Full cross-platform test surface depends on the CI ubuntu/macos runners: the CLI Tests jobs should confirm the gated helpers still compile for the test profile on all platforms (n/a locally — no Linux/macOS machine available; the platform matrix in CI is exactly the place this gets verified).

Reviewer Notes

• Three files, +1 attribute line each, zero deletions, zero behavior change.
• No #[allow] suppression is introduced.
• AI-assisted change, lightly tested (cargo check + targeted unit tests on Windows; cross-platform behavior verified by CI).

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable. (n/a — no user-facing change)