Fix frontmatter parameter interpolation in markdown scripts

[Copilot]

Fixes the issue where frontmatter parameters defined in markdown scripts were not being used for variable interpolation with {{ param }} syntax.

Problem

When markdown scripts defined parameters in frontmatter like this:

---
parameters:
  name:
    type: string
    default: "World"
---
Hello {{name}}!

The {{name}} variable was not being replaced with the default value "World" during interpolation. The frontmatter parameters were being parsed but never connected to the variable resolution system.

Root Cause

The interpolateVariables() function in mustache.ts was calling splitMarkdown() to extract content and discard frontmatter, meaning frontmatter parameters never made it into the variable interpolation process.

Solution

This PR modifies both mustache.ts and template.ts to properly extract and use frontmatter parameters during variable interpolation:

1. Enhanced interpolateVariables() to extract frontmatter parameters and merge them with provided data
2. Added support for both formats: standard parameters and prompty inputs + sample formats
3. Implemented proper precedence: provided data overrides frontmatter defaults
4. Maintained backward compatibility with existing functionality

Supported Formats

Standard format:

---
parameters:
  name:
    type: string
    default: "World"
  count:
    type: number
    default: 3
---
Hello {{name}}! This message will repeat {{count}} times.

Prompty format:

---
inputs:
  question:
    type: string
sample:
  "question": "What is the capital of France?"
---
Question: {{question}}

Key Features

• Frontmatter parameter defaults are used when no data is provided for that parameter
• Provided data takes precedence over frontmatter defaults
• Works with both Mustache and Jinja templating formats
• Supports both standard parameters and prompty inputs + sample formats
• Comprehensive test coverage for all scenarios including edge cases

Testing

Added 6 new test cases covering:

• Using frontmatter defaults when no data provided
• Partial data override scenarios
• Prompty format support
• Jinja templating compatibility
• Edge cases and backward compatibility

All existing functionality remains unchanged and working as expected.

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