test: Cover stateless GitHub App token format

[rajveer002]

Summary

Add focused regression coverage for the newer stateless GitHub App installation-token format.

The test uses a realistic long opaque token containing multiple segments and separators (. and _) and verifies that CreateInstallationToken preserves the token exactly, including its full length and content.

Why

The repository already has long-token coverage using repeated characters. This adds coverage for the structured stateless-token shape, helping guard against assumptions about token length or character layout.

Testing

• go test ./github -count=1 passed locally.
• git diff --check passed before commit.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.59%. Comparing base (48d0a66) to head (7bc7601).
⚠️ Report is 10 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4598   +/-   ##
=======================================
  Coverage   98.59%   98.59%           
=======================================
  Files         197      197           
  Lines       18326    18326           
=======================================
  Hits        18068    18068           
  Misses        258      258           

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[gmlewis]

@rajveer002 - once you fix the linter issues (you can run ./script/lint.sh locally) and push the changes, we can start moving forward with this PR.