Show ruleset violation details when create_branch fails

[Hashim1999164]

Summary

• create_branch currently returns a compact 422 dump when a repository rule rejects the new name
• Tool errors now include each nested validation message and the docs URL so agents can recover the way the GitHub UI already explains the rule

Why

Fixes #3080

What changed

• Format github.ErrorResponse in NewGitHubAPIErrorResponse so nested error messages are visible
• Cover a ruleset style 422 in the create_branch tests

MCP impact

• Tool schema or behavior changed
  Error text for GitHub REST failures now includes nested validation messages. Tool schema is unchanged.

Prompts tested (tool changes only)

• Create a branch named hotfix in a repo whose ruleset requires feature/* and confirm the tool error names the pattern

Security / limits

• No security or limits impact

Tool renaming

• I am not renaming tools as part of this PR

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test

Docs

• Not needed