refactor(runtime): extract tool schema planning and validation

[jackeyfaker77]

Summary

After #4779, tool schema planning and Code Mode input validation still live inside ai-sdk-turn.ts. Extract them into tool-schema-builder.ts so they can be tested independently, while preserving existing behavior and keeping execution and settlement in their current owners.

Refs #3909

Verification

Passed locally:

• Core, Storage, and Runtime builds
• Runtime typecheck
• Repository lint and format checks
• 53 tests across tool-schema-builder, code-mode-backend, tool-availability, and request-shape
• git diff --check

Note: The full Runtime suite on Windows encountered pre-existing SQLite file-lock cleanup (EBUSY) during test teardown; all target units and affected subsystem tests pass cleanly.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No