test: Support stateless GitHub App installation tokens

[sundaypaulfz3-cell]

Summary

Add regression coverage for GitHub App stateless installation tokens.

GitHub's newer stateless installation tokens can be significantly longer than legacy installation tokens. These tests verify that go-github treats installation tokens as opaque strings rather than imposing fixed-length assumptions.

Changes

• Add coverage for deserializing a long ghs_ installation token from the installation-token API response.
• Verify the complete token is preserved without truncation.
• Add coverage verifying that a long token passed through WithAuthToken is sent unchanged as:
  Authorization: Bearer <token>.
• Preserve existing authentication behavior for legacy tokens.

Testing

• go test ./github — passed
• git diff --check — passed
• go test -v -race ./github — unable to run because Go's race detector is not supported on Android/ARM64.

The current implementation already treats the token as an opaque string without a fixed-length validation constraint, so this change adds regression coverage for the required behavior.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.59%. Comparing base (8d9197f) to head (f910665).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4577   +/-   ##
=======================================
  Coverage   98.59%   98.59%           
=======================================
  Files         197      197           
  Lines       18326    18326           
=======================================
  Hits        18068    18068           
  Misses        258      258           

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