Use schemars for strict #[tool] parameter schemas

[luckenco]

Problem

#[tool] currently emits shallow JSON Schemas for tool parameters in macros/src/tool.rs.

Known weak spots:

• Vec<T> becomes only { "type": "array" }, without an items schema.
• Custom/complex parameter types become unconstrained { "type": "object" }.
• Optionality is detected only for bare Option<T>, missing paths like std::option::Option<T>.
• OpenAI strict mode is weaker than expected because the schema loses nested object, array item, enum, and nullability details.
• Explicit null for optional fields can fail during execution because the macro strips Option<T> before parameter deserialization.

Proposed Direction

• Generate a private params struct per tool.
• Derive serde::Deserialize and schemars::JsonSchema on that struct.
• Use docstring parameter descriptions as #[schemars(description = "...")].
• Deserialize tool call arguments into the generated params struct before invoking the user function.
• Require tool parameter types to implement both Deserialize and JsonSchema.
• Keep Tool.parameters provider-neutral and Rust-shaped.
• Normalize provider-specific schemas in adapters:
  • OpenAI/OpenRouter strict: recursively require all properties and set additionalProperties: false.
  • Gemini: preserve normal optional required semantics and convert schemars output into the Gemini supported schema subset.

Acceptance Criteria

• Vec<String> schemas include items.
• Nested structs/enums produce real schemas.
• Option<T> and std::option::Option<T> behave as optional Rust parameters.
• Missing and explicit null optional arguments both deserialize to None.
• OpenAI strict request schemas satisfy strict requirements recursively.
• Gemini request schemas preserve optional fields without forcing them into required.
• Unsupported custom parameter types fail clearly at compile time.

Suggested Tests

• Macro schema tests for arrays, nested structs, enums, and option paths.
• Execution tests for missing and null optional values.
• Provider conversion tests for OpenAI strict normalization and Gemini schema conversion.
• Compile-fail test for a custom parameter type without JsonSchema.