fix(config): route GitHub App token refresh to enterprise API_URL

[Ahmadshata]

Overview

This PR fixes GitHub App authentication for GitHub Enterprise environments by ensuring installation token refresh requests use the configured API_URL instead of defaulting to api.github.com.

Related Issues

• Resolves [BUG] GitHub App auth on GHES still uses api.github.com for installation token refresh instead of the configured API_URL #138
• Can't connect to private Github instance #132

Problem

When GITHUB_APP=true and API_URL points to GitHub Enterprise, API requests were correctly sent to the enterprise endpoint, but installation token refresh still targeted GitHub.com (/app/installations/{id}/access_tokens).
This caused authentication failures (401) in enterprise deployments.

Root Cause

ghinstallation.Transport was initialized, but its BaseURL was not overridden when a non-default API_URL was configured.

Changes

• Updated GetClient() in config/config.go to set:
  • itr.BaseURL = c.ApiUrl.String() when API_URL is not https://api.github.com.
• Added regression coverage in config/config_test.go:
  • New test verifies GitHub App transport BaseURL is set to enterprise API_URL.
  • Added a temporary private key test helper for GitHub App transport construction.

Validation

• go test ./config
• go test ./...

Impact

• Fixes GitHub App installation token refresh for GitHub Enterprise users.
• No behavior change for GitHub.com/default API_URL setups.
• No API or config interface changes.

Risk

Low.
The change is narrowly scoped to transport initialization for GitHub App auth and is covered by targeted regression testing.