Add fallback message for blank tool execution errors

[cho-thinkfree-com]

Summary

• return a descriptive fallback when ToolExecutionException#getMessage() is null or blank
• include the tool name and originating exception type so downstream LLMs can detect failures
• cover the new behavior with dedicated unit tests

Checklist

• Signed-off-by line added to the commit (git commit -s)
• Rebased on the latest main and squashed into a single commit
• Unit tests updated to cover the new behavior
• Build executed locally and tests pass

Testing

• ./mvnw -pl spring-ai-model -Dtest=DefaultToolExecutionExceptionProcessorTests test

Fixes #4518

[ilayaperumalg]

@cho-thinkfree-com Thanks for the PR addressing the issue! LGTM

[ilayaperumalg]

Rebased and merged as f2d57cc. Cherry-picked to 1.0.x as well.