fix(core): validate model name in /model command against provider list

[bug-ops]

Summary

• /model <id> now validates the provided model name against the cached provider model list before applying it
• Unknown models are rejected with an error message listing available options
• When the model list is unavailable (cold start or provider without listing support), a warning is shown and the switch proceeds

Changes

• crates/zeph-core/src/agent/mod.rs: validation in handle_model_command() — cache-first, remote fallback, early return on unknown model
• crates/zeph-llm/src/mock.rs: MockProvider gains optional models field and with_models() builder for test injection
• crates/zeph-llm/src/any.rs: AnyProvider::Mock returns injected models from list_models_remote(); doc comment updated

Test plan

• Valid model accepted — set_model called, no error
• Invalid model rejected — error message with available models, set_model not called
• Empty/unavailable model list — warning shown, switch proceeds (backward compatibility)
• cargo +nightly fmt --check clean
• cargo clippy --workspace --features full -- -D warnings clean
• cargo nextest run --workspace --features full --lib --bins — 4872 passed

Fixes #1417