fix(core): disable truncation when maxChars <= 0 in formatTruncatedToolOutput - #29542

Summary

Clarifies behavior and adds a non-positive guard so that maxChars <= 0 disables output truncation and returns contentStr unchanged, preventing index slicing edge cases where output string size inadvertently expanded.

Details

In packages/core/src/utils/fileUtils.ts, formatTruncatedToolOutput calculates head and tail character slices to display output previews when strings exceed maxChars. When maxChars <= 0, negative slice indices cause unexpected boundary behavior where head and tail slices capture nearly the full string, resulting in output size inflation.

This change updates the early return condition to:

if (maxChars <= 0 || contentStr.length <= maxChars) return contentStr;

When maxChars <= 0, truncation is treated as disabled and contentStr is returned untouched without wrapping in truncation headers.

Regression unit tests have been added to packages/core/src/utils/fileUtils.test.ts to verify behavior for maxChars = 0, maxChars < 0 (e.g., -1, -1000), and maxChars > 0.

Related Issues

Fixes #28620

How to Validate

1. Run unit tests for fileUtils:

   npx vitest run packages/core/src/utils/fileUtils.test.ts

   Verify all tests in formatTruncatedToolOutput pass, including cases for maxChars = 0 and maxChars < 0.
2. Run type checking and linting:

   npm run typecheck
   npx eslint packages/core/src/utils/fileUtils.ts packages/core/src/utils/fileUtils.test.ts

Pre-Merge Checklist

• Updated relevant documentation and README (if needed)
• Added/updated tests (if needed)
• Noted breaking changes (if any)
• Validated on required platforms/methods:
  • MacOS
    • npm run
    • npx
    • Docker
    • Podman
    • Seatbelt
  • Windows
    • npm run
    • npx
    • Docker
  • Linux
    • npm run
    • npx
    • Docker