fix(tools): reject tool calls missing schema-required arguments before dispatch

[JoaoMarcos44]

Summary

The normal dispatch path coerced args but never validated required fields, so a tool with required:["x"] executed with {}. Adds a fail-open required-key-absence check after coercion/middleware and before authorization/side effect.

Changes

• model_tools._validate_tool_args_against_schema: checks parameters.required key absence (same contract as validate_deferred_call_args), returns tool_error(..., effect_disposition="not_started", retryable=True).
• Wired into handle_function_call after the agent-loop check, before the pre_tool_call hook.
• Updated 5 test_model_tools calls and 1 test_sanitize_tool_error call that used "q" as a placeholder for web_search's required query field.

Test Plan

• New test_missing_required_arg_is_rejected_before_dispatch (reject {} with not_started, allow valid call).
• scripts/run_tests.sh on model_tools + async bridge + tool_search + sanitize_tool_error + transform hook → 51/51.

Closes #84689

[alt-glitch]

This was generated by AI during triage.

Duplicate of #61550 — both add required-key-only JSON-Schema validation to normal model-tool dispatch before side effects. #61550 is the earlier, broader implementation and also validates final execution boundaries.