Fix subdomain isolation URL parsing

[tonytrg]

Fixes issues in GHES when subdomain isolation is enabled.

PR introduces:

• raw api probe to detect subdomain isolation
  • changes url format for affected urls in func newGHESHost(hostname string) (apiHost, error)
• changes the return message of get_file_contents to return better fallback message
  • better context for llm

Closes:

• get_file_contents is not working properly (Path did not point to a file or directory, but resolved git ref) #864

[Copilot]

Pull Request Overview

This PR fixes URL parsing for GitHub Enterprise Server (GUES) instances with subdomain isolation enabled. The fix addresses issues where the server was incorrectly constructing URLs for raw content and upload APIs when subdomain isolation was configured.

• Adds automatic detection of subdomain isolation by probing the raw subdomain endpoint
• Updates URL construction logic to use subdomain-based URLs when isolation is detected
• Improves error messages to provide better context when raw API calls fail

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
internal/ghmcp/server.go	Implements subdomain isolation detection and conditional URL formatting logic
pkg/github/repositories.go	Captures raw API response codes and improves error messaging for better debugging context

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[nasxisbest]

Great work, thank you! Love the new feedback for LLMs.