fix: use ClientSecretCredential instead of DefaultAzureCredential in graph-connector template

[Copilot]

Cherry-pick of PR #15400 to dev. The graph-connector template was using DefaultAzureCredential with a non-standard requiredEnvVars option, which doesn't reliably authenticate with a service principal. Replace it with explicit ClientSecretCredential.

Changes

• templates/vsc/ts/graph-connector/src/graphClient.ts
  • Swap DefaultAzureCredential → ClientSecretCredential
  • Pass AZURE_TENANT_ID, AZURE_CLIENT_ID, AZURE_CLIENT_SECRET explicitly

// Before
const credential = new DefaultAzureCredential({
  requiredEnvVars: ["AZURE_TOKEN_CREDENTIALS"],
});

// After
const credential = new ClientSecretCredential(
  process.env.AZURE_TENANT_ID!,
  process.env.AZURE_CLIENT_ID!,
  process.env.AZURE_CLIENT_SECRET!
);

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Cherry-pick] PR #15400 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 #15400 (fix: not use DefaultAzureCredential but use ClientSecretCredential) is being merged into the release/6.5 branch.

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

• cherry-pick fix: not use DefaultAzureCredential but use ClientSecretCredential #15400 to dev
• cherry-pick fix: not use DefaultAzureCredential but use ClientSecretCredential #15400 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 #15400 needs to be synchronized to other branches #15406

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.30%. Comparing base (5e96d3e) to head (effc274).
⚠️ Report is 4 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev   #15416   +/-   ##
=======================================
  Coverage   89.30%   89.30%           
=======================================
  Files         620      620           
  Lines       38089    38089           
  Branches     7557     7559    +2     
=======================================
  Hits        34017    34017           
  Misses       1972     1972           
  Partials     2100     2100           

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