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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