fix(compact): use saturating subtraction to prevent underflow

[echobt]

Summary

Fixes #5096 and #5226 - Arithmetic underflow in compaction calculations.

Problem

Token count subtractions can underflow when removed tokens exceed original count.

Solution

Used saturating_sub() for safe arithmetic without panic or wrap.

[greptile-apps]

Greptile Overview

Greptile Summary

Replaced unsafe arithmetic operations with saturating arithmetic to prevent integer underflow in token count calculations during conversation compaction.

Key Changes:

• Changed current_tokens - tokens_in_compacted + summary_tokens to use saturating_sub() and saturating_add()
• Prevents panic when tokens_in_compacted exceeds current_tokens (which could happen due to token estimation inaccuracies)
• Maintains correctness by clamping to 0 instead of wrapping around on underflow

Safety Improvement:
The fix aligns with Rust best practices seen throughout the codebase, where saturating arithmetic is used extensively for UI calculations and other scenarios where underflow is possible.

Confidence Score: 5/5

• This PR is safe to merge with no risk
• The change is minimal, well-documented, and follows established patterns in the codebase. It replaces potentially unsafe arithmetic with saturating operations, which is a defensive programming practice that prevents panics without changing the logical behavior in normal circumstances
• No files require special attention

Important Files Changed

Filename	Overview
src/cortex-compact/src/compactor.rs	replaced unsafe arithmetic with saturating_sub and saturating_add to prevent integer underflow in token count calculations

Sequence Diagram

sequenceDiagram
    participant Client
    participant Compactor
    participant Summarizer
    
    Client->>Compactor: compact(items, summary_text, current_tokens)
    Compactor->>Summarizer: select_items_to_compact(items)
    Summarizer-->>Compactor: items to compact
    
    alt No items to compact
        Compactor-->>Client: no_compaction result
    else Items to compact
        Compactor->>Compactor: Calculate tokens_in_compacted
        Compactor->>Summarizer: estimate_tokens(summary_text)
        Summarizer-->>Compactor: summary_tokens
        
        Note over Compactor: Build new items list with summary
        
        Note over Compactor: Calculate tokens_after using<br/>saturating_sub & saturating_add<br/>to prevent underflow
        
        Compactor->>Compactor: tokens_after = current_tokens<br/>.saturating_sub(tokens_in_compacted)<br/>.saturating_add(summary_tokens)
        
        Compactor-->>Client: CompactionResult with new items
    end

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[greptile-apps]

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[echobt]

Consolidated into #71 - fix: consolidated numeric overflow/underflow prevention with saturating operations