Fix getUser() 404 for bot users in PR reviews

[Haarolean]

Description

Fixed getUser() 404 for bot users in PR reviews.

Use intern() instead of getUser() to avoid API calls to /users/{login}, which returns 404 for bot/app users. This matches the pattern already used by GHIssue, GHRepositoryDiscussion, and others.

Verified locally consuming a related webhook.

original stacktrace:

java.io.FileNotFoundException: https://api.github.com/users/Copilot
    at org.kohsuke.github.GitHubConnectorResponseErrorHandler$1.onError(GitHubConnectorResponseErrorHandler.java:60)
    at org.kohsuke.github.GitHubClient.detectKnownErrors(GitHubClient.java:504)
    at org.kohsuke.github.GitHubClient.sendRequest(GitHubClient.java:464)
    at org.kohsuke.github.GitHubClient.sendRequest(GitHubClient.java:427)
    at org.kohsuke.github.Requester.fetch(Requester.java:85)
    at org.kohsuke.github.GitHub.getUser(GitHub.java:601)
    at org.kohsuke.github.GHPullRequestReviewComment.getUser(GHPullRequestReviewComment.java:133)
<..>
org.kohsuke.github.GHFileNotFoundException: https://api.github.com/users/Copilot {"message":"Not Found","documentation_url":"https://docs.github.com/rest","status":"404"}
    at org.kohsuke.github.GitHubClient.interpretApiError(GitHubClient.java:661)
    at org.kohsuke.github.GitHubClient.sendRequest(GitHubClient.java:480)
    at org.kohsuke.github.GitHubClient.sendRequest(GitHubClient.java:427)
    at org.kohsuke.github.Requester.fetch(Requester.java:85)
    at org.kohsuke.github.GitHub.getUser(GitHub.java:601)
    at org.kohsuke.github.GHPullRequestReviewComment.getUser(GHPullRequestReviewComment.java:133)
<..>

Before submitting a PR:

• Changes must not break binary backwards compatibility. If you are unclear on how to make the change you think is needed while maintaining backward compatibility, CONTRIBUTING.md for details.
• Add JavaDocs and other comments explaining the behavior.
• When adding or updating methods that fetch entities, add @link JavaDoc entries to the relevant documentation on https://docs.github.com/en/rest .
• Add tests that cover any added or changed code. This generally requires capturing snapshot test data. See CONTRIBUTING.md for details.
• Run mvn -D enable-ci clean install site "-Dsurefire.argLine=--add-opens java.base/java.net=ALL-UNNAMED" locally. If this command doesn't succeed, your change will not pass CI.
• Push your changes to a branch other than main. You will create your PR from that branch.

When creating a PR:

• Fill in the "Description" above with clear summary of the changes. This includes:
  • If this PR fixes one or more issues, include "Fixes #" lines for each issue.
  • Provide links to relevant documentation on https://docs.github.com/en/rest where possible. If not including links, explain why not.
• All lines of new code should be covered by tests as reported by code coverage. Any lines that are not covered must have PR comments explaining why they cannot be covered. For example, "Reaching this particular exception is hard and is not a particular common scenario."
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.03%. Comparing base (9152b37) to head (4fe5610).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2207      +/-   ##
============================================
- Coverage     85.03%   85.03%   -0.01%     
+ Complexity     2499     2498       -1     
============================================
  Files           237      237              
  Lines          7366     7364       -2     
  Branches        388      388              
============================================
- Hits           6264     6262       -2     
  Misses          871      871              
  Partials        231      231              

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