validate labels before creating issue to prevent duplicates #13

bug report#

when creating an issue with invalid labels, the mcp server currently:

creates the issue successfully
then returns an error about invalid labels
this causes the caller to retry, creating duplicate issues

example#

> create_repo_issue(repo="zzstoatzz.io/find-bufo", title="...", labels=["enhancement"])
Error: invalid labels: ['enhancement']
available labels for this repo: ['assignee', 'documentation', 'duplicate', 'good-first-issue', 'wontfix']

this error message suggests the operation failed, but the issue was actually created. retrying creates a duplicate.

expected behavior#

the server should validate labels against the repo's available labels before creating the issue. if labels are invalid, fail fast without creating the issue.

actual behavior#

the server creates the issue first, then validates labels and returns an error, leaving the issue created even though an error was returned.

impact#

confusing developer experience (error returned but operation succeeded)
creates duplicate issues when retried
wastes issue numbers

fix#

validate labels parameter against repo's available labels before calling the issue creation api.