Fix critical bugs in string truncation and context window lookup

[pavankumar-vh]

Overview

Fix two critical bugs that could cause runtime errors and improve code safety.

Bug Fixes

1. Fix context window lookup in base-chat.ts

Issue: The code used CONTEXT_WINDOWS[model ?? ''] which converts undefined model to empty string and unnecessarily performs a lookup.

Fix: Changed to (model && CONTEXT_WINDOWS[model]) ?? DEFAULT_CONTEXT_WINDOW which:

• Avoids unnecessary lookup when model is undefined
• More clearly expresses intent
• Falls back to default window correctly

2. Fix critical bug in truncateStringWithMessage

Issue: When maxLength is smaller than the message length, the function calculates negative slice indices, causing unexpected behavior.

Fix: Added Math.max(0, ...) guards to prevent negative slice lengths:

• END mode: Math.max(0, maxLength - suffix.length)
• START mode: Math.max(0, maxLength - prefix.length)
• MIDDLE mode: Math.max(0, Math.floor((maxLength - middle.length) / 2))

This prevents potential runtime errors when truncating strings with very small maxLength values.

3. Add comprehensive tests for truncateStringWithMessage

Added 9 test cases covering:

• All truncation modes (END, START, MIDDLE)
• Edge cases with negative/zero available length
• Custom message handling
• Empty string handling
• Default behavior verification

Files Changed

• agents/base-chat.ts - Improved context window lookup logic
• common/src/util/string.ts - Added safety guards to prevent negative slice indices
• common/src/util/__tests__/string.test.ts - Added comprehensive test coverage

Scope

This change only touches agents/ and common/ which are approved contribution areas per the Contributing Guide.