[codex] Add core tool call contract validation

[gold-silver-copper]

Summary

Adds the PR1 core validation layer for Rig tool calls without changing provider request serializers.

• Adds a request-side ToolCallContract, provider capability profile, and validation errors for invalid or unknown tool calls.
• Validates completion requests before dispatch so unsupported tool_choice settings and impossible forced-tool policies fail before provider calls.
• Validates provider-emitted tool calls before prompt hooks or local tool execution, including streaming deltas.
• Prevalidates all local tool calls before starting non-streaming buffered tool execution so a valid side-effecting tool cannot run before an unknown tool error.
• Adds focused tests around undeclared tools, disallowed tools, provider-native function declarations, hosted provider tools, required-without-tools, non-streaming prevalidation, and streaming malformed tool-call deltas.

Why

PR 27 had grown provider-specific serialization fixes and core contract behavior into one large change. This PR isolates the core contract and execution-safety layer so provider serializer/capability alignment can be handled separately in PR2.

Validation

• cargo fmt
• cargo test -p rig-core --lib
• cargo check -p rig-core
• git diff --check