fix: resolve empty arguments for Bedrock native tool calls

[IgnazioDS]

Summary

• Fix _parse_native_tool_call to correctly read Bedrock tool arguments
• The or-chain fallback was broken because get("arguments", "{}") returns a truthy default, preventing fallthrough to tool_call.get("input") where Bedrock places arguments

Problem

All AWS Bedrock models (Nova, Claude via Bedrock) always receive empty {} arguments when using native tool calling. Two independent issues report this: #4972 and #4748.

Fix

# Before (broken)
func_info.get("arguments", "{}") or tool_call.get("input", {})

# After (fixed)  
func_info.get("arguments") or tool_call.get("input") or "{}"

Test plan

• Verify OpenAI-format tool calls still parse correctly
• Verify Bedrock-format tool calls now receive proper arguments
• Verify fallback to "{}" works when neither key is present

Fixes #4972, Fixes #4748

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Note

Low Risk
Low risk, single-line change in native tool-call parsing; main concern is any provider relying on the previous default "{}" masking missing arguments.

Overview
Fixes native tool-call argument parsing in CrewAgentExecutor._parse_native_tool_call so dict-style calls (notably AWS Bedrock) correctly fall back from function.arguments to the top-level input payload instead of always defaulting to an empty {}.

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