fix(reposerver): using timeouts in http transport for Github App authentication (#26762)

[anandf]

Cherry pick of PR #26762

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[themavik]

This PR applies proper timeouts to the HTTP transport used for GitHub App auth by cloning DefaultTransport and using gitClientTimeout for getAccessToken.

Done well: Using http.DefaultTransport.Clone() preserves Go default timeouts. The creds.go change to use gitClientTimeout keeps behavior configurable. The new tests (TLS handshake timeout, custom timeout, dialer timeout) are thorough.

Note: The TestGitHubAppGetAccessToken_CustomTimeout test documents a known limitation: gitClientTimeout may not fully govern token requests if ghinstallation uses the transport without request context. Worth confirming the transport timeouts actually apply to the token endpoint.