fix: add missing $schema to declarative agent MCP template (EmbeddedKnowledge path)

[Copilot]

The {{#EmbeddedKnowledgeEnabled}} block in the declarative agent MCP action template was missing the $schema field, causing app manifest schema validation to fail for DA projects with MCP action when EmbeddedKnowledge (v1.6) is enabled.

Change

• templates/vsc/common/declarative-agent-with-action-from-mcp/appPackage/declarativeAgent.json.tpl: Added the missing $schema to the EmbeddedKnowledgeEnabled branch, matching the pattern already present in the non-embedded branch:

  {{#EmbeddedKnowledgeEnabled}}
+ "$schema": "https://developer.microsoft.com/json-schemas/copilot/declarative-agent/v1.6/schema.json",
  "version": "v1.6",
  {{/EmbeddedKnowledgeEnabled}}

Cherry-pick of #15375 to dev.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Cherry-pick] PR #15375 needs to be synchronized to other branches</issue_title>
<issue_description>

Cherry-pick Reminder

@Alive-Fish Hello!

This is an automatically created reminder. Your PR #15375 (fix: no schema of declarative agent in mcp server) is being merged into the release/6.5 branch.

Please ensure to cherry-pick these changes to the following branches:

• cherry-pick fix: no schema of declarative agent in mcp server #15375 to dev
• cherry-pick fix: no schema of declarative agent in mcp server #15375 to release/vs1714p7

Steps to Follow:

1. Wait for the current PR to be merged.
2. Execute the following commands locally:

   git fetch origin
   git checkout <target_branch>
   git cherry-pick <commit_hash>
   git push origin <target_branch>

3. Or create a new PR to merge these changes into the above branches.

After completion, please check the items above ✓

Note: This is an automatically created issue. You can close this issue after completing all the operations.
</issue_description>

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

• Fixes [Cherry-pick] PR #15375 needs to be synchronized to other branches #15377

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.29%. Comparing base (c92a365) to head (e671139).
⚠️ Report is 6 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev   #15378   +/-   ##
=======================================
  Coverage   89.29%   89.29%           
=======================================
  Files         620      620           
  Lines       38065    38065           
  Branches     7723     7687   -36     
=======================================
  Hits        33989    33989           
  Misses       1973     1973           
  Partials     2103     2103           

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