Replace undefined scenario call lines with detailed diagnostic errors

[Copilot]

When scenario call expansion fails (missing feature/scenario, exceptions), the original call line was left in the generated output, appearing as an undefined step (purple) in tooling. Users received only a generic warning without understanding why expansion failed.

Changes

• Remove undefined call lines: Original scenario call lines are now excluded from output when expansion fails, preventing undefined step errors
• Diagnostic messages: Return specific error context instead of null:
  • Feature file not found: includes search path guidance
  • Scenario not found: suggests checking spelling/case
  • Exceptions: surfaces the error message
• New method: FindScenarioStepsWithDiagnostics returns (steps, diagnosticMessage) tuple instead of just steps

Example

Before:

Scenario: Test
    Given I call scenario "Missing" from feature "Unknown"
    # Warning: Could not expand scenario call
    Then I continue

The undefined call line remains, causing purple highlighting.

After:

Scenario: Test
    # ERROR: Could not find feature file for "Unknown". Ensure the feature file exists in the project or referenced projects.
    Then I continue

Call line removed, actionable diagnostic provided.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Scenario call line remains purple (undefined) due to missing/failed expansion</issue_title>
<issue_description>Lines like Given I call scenario "OpgaveDieEenDetailRegelBevatOpenen" from feature "Opgave" remain highlighted in purple in the editor, indicating the line is considered an undefined step.

Current situation:

• The design intent of this plugin is that scenario call lines are expanded at generation/preprocessing time, so that the call line itself should not require a step definition.
• If expansion fails (e.g., scenario not found, feature not found, dialect mismatch, unsupported context such as Background, etc.), the original call line can remain in the generated/processed output and therefore appears as “undefined” (purple) in tooling.

Expected functionality:

• Scenario call lines should either:
  • Always expand (so the call line never remains), OR
  • If expansion cannot be performed, produce a very explicit diagnostic explaining why (feature not found, scenario not found, language mismatch, background limitation, etc.) so users can fix it quickly.

References (where expansion happens / fails):

• Expansion logic:
  https://github.com/jgerits/Reqnroll.ScenarioCall.Generator/blob/main/Reqnroll.ScenarioCall.Generator/ScenarioCallFeatureGenerator.cs#L228-L246
• Scenario lookup logic:
  https://github.com/jgerits/Reqnroll.ScenarioCall.Generator/blob/main/Reqnroll.ScenarioCall.Generator/ScenarioCallFeatureGenerator.cs#L269-L294
• README error handling guidance:
  https://github.com/jgerits/Reqnroll.ScenarioCall.Generator/blob/main/README.md#L174-L204

Implementation hint:

• Add a “strict mode” option (or default behavior) that fails generation when a scenario call cannot be resolved, rather than leaving the original call line behind.
• Alternatively, append a detailed comment right above the unexpanded line with the exact resolution failure reason.</issue_description>

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

• Fixes Scenario call line remains purple (undefined) due to missing/failed expansion #30

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