Add implementation plan for handlebar template support in Dapr PubSub reaction

[Copilot]

Description

Creates comprehensive implementation plan for adding handlebar template support to the Dapr PubSub reaction, enabling custom output formatting similar to Sync Vector Store and MCP reactions.

Plan Contents

• Current State Analysis: Surveyed existing handlebar implementations in Sync Vector Store (Handlebars.Net) and MCP (TypeScript) reactions
• Proposed Solution: Add Templated output format with separate templates for added, updated, and deleted results
• Implementation Details:
  • QueryConfig extension with addedTemplate, updatedTemplate, deletedTemplate properties
  • New TemplateChangeFormatter class with template caching
  • ChangeFormatterFactory updates to route templated events
  • Validation ensuring templates match format setting
• Testing Strategy: Unit tests for formatter, caching, validation; integration tests for end-to-end flow
• Documentation: Usage examples, context variables, helper functions
• Backward Compatibility: Opt-in feature, existing Packed/Unpacked formats unchanged

Template configuration example:

queries:
  inventory-changes: |
    {
      "pubsubName": "drasi-pubsub",
      "topicName": "inventory-events",
      "format": "Templated",
      "addedTemplate": "{\"type\": \"inventory_added\", \"product\": \"{{after.productName}}\", \"quantity\": {{after.quantity}}}",
      "updatedTemplate": "{\"type\": \"inventory_updated\", \"product\": \"{{after.productName}}\", \"oldQty\": {{before.quantity}}, \"newQty\": {{after.quantity}}}"
    }

Plan document: /plans/issue-345-dapr-pubsub-handlebar-templates.md

Type of change

• This pull request adds or changes features of Drasi and has an approved issue (issue link required).

Fixes: #345

Original prompt

go and create a plan to implement the feature requested in this issue #345. look at other reactions that currently support handlebar templates as guidance. write the plan as a markdown document in the /plans folder of this repo.

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