Flatten Tool metadata API

[gold-silver-copper]

Summary

• flatten Tool and ToolDyn metadata to name / description / parameters
• remove prompt-dependent definition(...) from tool authoring and erased runtime traits
• generate provider-facing ToolDefinitions at registry/request boundaries from registered tool names
• update #[rig_tool], MCP/vector-store tools, examples, provider tests, and changelogs

Migration

Tool authors now write:

const NAME: &'static str = "search";

fn description(&self) -> String {
    "Search docs".into()
}

fn parameters(&self) -> serde_json::Value {
    serde_json::json!({ /* schema */ })
}

instead of returning a ToolDefinition from definition(prompt).

Verification

• cargo fmt
• cargo check -p rig-core --all-targets --all-features
• cargo check --workspace --all-targets --all-features
• cargo clippy --all-targets --all-features
• cargo test -p rig-core --lib
• cargo test -p rig-derive
• cargo test --doc --workspace --all-features