refactor(core)!: simplify tool and hook APIs

[gold-silver-copper]

Summary

• replace parallel tool execution APIs with one canonical Tool::call(&mut ToolContext, args) path and one structured ToolExecutionError envelope
• hide erased dispatch behind DynamicTool, collapse tool/result metadata into ToolContext, and expose one structured dispatch method on ToolSet and ToolServerHandle
• replace universal Flow/StepEvent hook dispatch with event-specific lifecycle methods and action types, preserving patch merging, rewrite chaining, nested stacks, and run/stream parity
• migrate MCP, derive macros, providers, builders, examples, docs, and tests to the replacement API with no compatibility aliases
• preserve provider definitions, multimodal output, framework skip vs tool refusal, MCP metadata/error classification, and presentation-only result rewrites

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features
• cargo check --workspace --all-targets --all-features
• cargo check -p rig-core --no-default-features --features wasm,rmcp --target wasm32-unknown-unknown
• cargo test -p rig-core --all-features agent:: (275 passed, 2 ignored)
• cargo test -p rig-core --all-features tool:: (43 passed)
• cargo test -p rig-derive --all-features
• OpenAI request-hook, permission-control, and typed-tool cassette tests
• Gemini hook-stress and multimodal tool-result cassette tests
• Anthropic tool-result rewrite cassette tests

cargo doc -p rig-core --no-deps is blocked by a Rust 1.94.0 stringdex search-index ICE. A full rig-core run also has one unchanged Gemini image-generation baseline failure; all 1,228 other tests passed (8 ignored), and focused changed-surface suites are green.

Addresses #2126