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<h2>Greptile Overview</h2>

<h3>Greptile Summary</h3>


Replaces `panic!()` and `unreachable!()` macros throughout the helixc
compiler with proper error handling using descriptive error messages.

## Key Changes
- **Added two new error codes**: E627 for shortest path algorithms
missing required `from`/`to` parameters, and E628 for invalid `DROP`
usage on non-traversals
- **Replaced 4 panics in graph step validation**: ShortestPath,
ShortestPathDijkstras, ShortestPathBFS, and ShortestPathAStar now return
E627 errors instead of panicking when both `from` and `to` are missing
- **Replaced panic in statement validation**: `DROP` statement now
generates E628 error when applied to non-traversal expressions
- **Replaced 2 panics in traversal validation**: Invalid field value
types now generate E206 errors with `GeneratedValue::Unknown` fallback
- **Replaced 15+ `unreachable!()` calls in parser**: All parser methods
now return descriptive `ParserError` messages for unexpected grammar
rules

## Impact
This PR significantly improves compiler robustness by ensuring invalid
HelixQL queries produce helpful error messages instead of crashing the
compiler. All panic paths have been replaced with proper error
propagation that provides context to users about what went wrong.

<details><summary><h3>Important Files Changed</h3></summary>



File Analysis



| Filename | Score | Overview |
|----------|-------|----------|
| helix-db/src/helixc/analyzer/error_codes.rs | 5/5 | Added two new
error codes (E627 for missing shortest path parameters, E628 for invalid
DROP usage) with proper documentation and implementations |
| helix-db/src/helixc/analyzer/methods/graph_step_validation.rs | 5/5 |
Replaced four `panic!()` calls with proper E627 error generation for
shortest path algorithms missing both from/to parameters |
| helix-db/src/helixc/analyzer/methods/statement_validation.rs | 5/5 |
Replaced panic with E628 error when DROP is applied to non-traversal
expressions |
| helix-db/src/helixc/parser/expression_parse_methods.rs | 5/5 |
Replaced three `unreachable!()` macros with descriptive error messages
for unexpected parser rules |
| helix-db/src/helixc/parser/graph_step_parse_methods.rs | 5/5 |
Replaced four `unreachable!()` macros with proper ParserError for
unexpected rules in order_by and shortest path parsing |

</details>


</details>


<details><summary><h3>Sequence Diagram</h3></summary>

```mermaid
sequenceDiagram
    participant User
    participant Parser
    participant Analyzer
    participant ErrorHandler
    
    Note over Parser,Analyzer: Before: panics on unexpected input
    
    User->>Parser: Submit HelixQL query
    
    alt Unexpected parse rule encountered
        Parser->>Parser: Match rule
        Parser-->>ErrorHandler: Return ParserError with context
        ErrorHandler-->>User: Descriptive error message
    end
    
    alt Valid parse, needs analysis
        Parser->>Analyzer: Pass parsed AST
        
        alt ShortestPath missing from/to
            Analyzer->>Analyzer: Validate graph step
            Analyzer-->>ErrorHandler: generate_error!(E627)
            ErrorHandler-->>User: "requires either `from` or `to` parameter"
        end
        
        alt DROP on non-traversal
            Analyzer->>Analyzer: Validate DROP statement
            Analyzer-->>ErrorHandler: generate_error!(E628)
            ErrorHandler-->>User: "DROP can only be applied to traversals"
        end
        
        alt Invalid field value type
            Analyzer->>Analyzer: Validate field values
            Analyzer-->>ErrorHandler: generate_error!(E206)
            ErrorHandler-->>User: "invalid value type"
        end
    end
    
    Note over Parser,ErrorHandler: After: graceful error handling
```
</details>


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