Update x-copy behavior to require single source match per specification PR #150

[Copilot]

Overview

This PR implements the x-copy behavior changes from OAI Overlay Specification PR #150, which requires that the copy JSON path expression must match exactly one result, and all targets receive the same source value.

Changes

Behavioral Changes

Previous behavior:

• Copy JSON path could match multiple results
• Targets and copy sources were paired sequentially (1-to-1 matching)
• Example: If target matched 2 nodes and copy matched 2 nodes, they were paired as target[0]←copy[0], target[1]←copy[1]

New behavior:

• Copy JSON path must match exactly one result
• All matching targets receive the same source value
• Example: If target matches 3 nodes and copy matches 1 node, all targets receive that single source: target[0]←copy[0], target[1]←copy[0], target[2]←copy[0]

Implementation Details

Modified CopyNodes method in src/lib/Models/OverlayAction.cs:

• Added validation to ensure copyParseResult.Matches.Count == 1
• Removed sequential pairing logic
• Simplified copy operation to use single source for all targets
• Updated error messages to clearly indicate exact match requirement

// New validation
if (copyParseResult.Matches.Count != 1)
{
    overlayDiagnostic.Errors.Add(new OpenApiError(GetPointer(index), 
        $"Copy JSON Path '{Copy}' must match exactly one result, but matched {copyParseResult.Matches.Count}"));
    return false;
}

// Simplified copy logic - single source to all targets
foreach (var match in matchValues)
{
    MergeJsonNode(match!, copyMatch, overlayDiagnostic);
}

Test Updates

Updated existing tests (4):

• Modified ApplyToDocument_ShouldCopyMultipleMatches to test single source → multiple targets
• Renamed and repurposed ApplyToDocument_CopyShouldFailWhenMatchCountsDiffer to ApplyToDocument_CopyShouldFailWhenCopyHasMultipleMatches
• Updated error message expectations in ApplyToDocument_CopyShouldFailWhenCopyTargetHasNoMatches and ApplyToDocument_CopyShouldFailWhenCopyTargetNotFound

Added new tests (4):

• ApplyToDocument_CopyShouldCopySingleSourceToMultipleTargets - Validates copying single source to 3 targets
• ApplyToDocument_CopyShouldFailWhenCopyMatchesZeroResults - Validates rejection when copy path matches 0 results
• ApplyToDocument_CopyShouldFailWhenCopyMatchesTwoResults - Validates rejection when copy path matches 2 results
• ApplyToDocument_CopyShouldSucceedWithExactlyOneMatch - Validates success with exactly 1 match

Testing

• ✅ All 55 tests passing (53 library + 2 CLI)
• ✅ CodeQL security scan: 0 vulnerabilities
• ✅ Clean build with no warnings

Breaking Change

This is a breaking change for users who rely on the previous sequential copy behavior with multiple copy source matches. The copy JSON path expression must now match exactly one result.

Closes #[issue_number]

Original prompt

---

This section details on the original issue you should resolve

<issue_title>x-copy behavior changes</issue_title>
<issue_description>The specification PR OAI/Overlay-Specification#150 is making good progress.

One of the notable behavior changes is that the copy JSON path expression MUST only match a single object/array, and there's no sequential copy of copy results to target results anymore. (all targets will use the same source)

We need to update the implementation to reflect that:

• throw an error of the copy json path expression results in anything else than a single result.
• add unit tests to validate that behavior.
• replace the sequential copy in copy nodes by a single source
• update impacted unit tests</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #120

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