test: Enhance test coverage for tool execution components

[alxkm]

Description

This PR enhances test coverage for the tool execution components in Spring AI, focusing on edge cases and boundary conditions that were previously untested.

Changes

DefaultToolExecutionResult Tests

• Added test for immutable list handling in conversation history
• Added test for multiple returnDirect() calls in builder pattern

ToolExecutionEligibilityPredicate Tests

• Added test for empty generations in ChatResponse
• Added test for model-based predicate with ModelCheckingPredicate implementation

ToolExecutionResult Tests

• Added test for empty conversation history throwing exception
• Added test for empty tool response list
• Added test for null content in tool response
• Added test for empty string content in tool response
• Added test for builder without conversation history

These tests improve code reliability by

• Increases test coverage for tool execution components
• Covers previously untested edge cases and boundary conditions
• Ensures robust error handling for invalid inputs