fix(messaging): prefer flow main when select_app_flow is ambiguous

[BimaPangestu28]

Summary

• select_app_flow errored with APP_FLOW_NOT_RESOLVED whenever an app pack shipped >1 messaging flow without an explicit default flow.
• Adds a main fallback (restricted to kind: messaging) between the existing default-name lookup and the single-messaging-flow heuristic.
• No behavior change for single-flow packs or packs that already define a default flow.

Why

Multi-flow app packs were silently broken. Concrete repro:

supply-chain-demo ships five messaging flows (main, on_message, order_tracking_flow, reorder_flow, stock_check_flow). When the user opens the webchat URL for that bundle and types anything, the runtime logs:

[Error] [demo ingress] messaging pipeline failed provider=messaging-webchat-gui err=select app default flow

…and no flow ever runs (state/runs/messaging/ stays empty, flow.log is empty, parsed events=0 in operator log even though the activity arrived).

The main flow already carries the entrypoint the wizard intends to surface (entrypoints: { default: "show_welcome" }). Treating it as the natural fallback matches the convention gtbundle already uses when it scaffolds packs.

Test plan

• cargo test -p greentic-start --lib select_app_flow — local: 4/4 pass (2 new, 2 existing unchanged)
• cargo clippy -p greentic-start --all-targets -- -D warnings — clean locally
• cargo fmt -p greentic-start --check — clean locally
• Smoke: rebuild greentic-start against this branch, install over ~/.local/bin/greentic-start, then gtc start supply-chain-demo-bundle → user message in webchat → verify flow main runs (welcome card appears) instead of erroring with select app default flow.
• Regression check: re-run gtc start hr-onboarding-demo-bundle → still resolves to main via the single-messaging-flow path (no behavior change).

Related

• greentic-runner #303 / #304 fix the adaptive-card render side (component invocation gets a valid card_spec); together with this PR, the supply-chain-demo bundle should render its welcome card end-to-end.