fix(dash): discover served model for configured chat endpoint

[volen-silo]

Summary

• The dash TUI chat now queries /v1/models at startup to discover the served model name when a chat endpoint is configured via OPENAI_BASE_URL / --chat-url but no model is set.
• Why: the documented rocm serve … && OPENAI_BASE_URL=… rocm flow was broken — sending a message returned 404 Not Found: The model 'local-model' does not exist, making chat unusable out of the box.
• Root cause: the managed-service and in-TUI detect-offer paths already discover the served model, but the startup env/CLI path fell straight through to resolve_llm_config's DEFAULT_CHAT_MODEL = "local-model" placeholder and built the agent with it. Accepting the consent prompt doesn't rebuild, so the placeholder reached the first request.
• Fix: after resolving the config, when the endpoint is reachable and no explicit model was given, fetch /v1/models and adopt the served id (via the existing fetch_first_model / pick_first_model helpers), mirroring the managed-service path.

Non-obvious decisions

• Gated on a successful TCP probe (probe_ok) so an unreachable configured endpoint never adds the 3s fetch timeout to startup.
• An explicit --chat-model / config value always wins — discovery only fills the placeholder (enforced by the pure with_discovered_model helper).
• Kept the local-model fallback when /v1/models can't be read — some endpoints ignore the model field or don't expose the listing.

Test plan

• cargo test -p rocm-dash-tui --lib — 549 passed (3 new unit tests for with_discovered_model: placeholder replaced, explicit model preserved, failed discovery keeps fallback).
• cargo clippy -p rocm-dash-tui --all-targets — clean.
• Manual (deferred, needs a live engine): rocm serve Qwen/Qwen2.5-1.5B-Instruct --engine vllm, then OPENAI_BASE_URL=http://127.0.0.1:11435/v1 rocm, accept the endpoint in the Chat tab, send a message → expect a normal reply and no local-model 404.

Risk

Low — additive best-effort discovery on a single startup path; existing precedence and the neutral fallback are preserved.