JIRA diagnostics: hedge as likely causes, add two-URL mismatch hint to inconclusive arms

Per review of #152: a failed accessible-resources probe is not proof the
credentials are bad (the endpoint may not accept basic auth for every
token type), so the 401/403 arm no longer asserts "credentials rejected"
- it now enumerates likely causes. Every inconclusive arm (401/403,
unreachable, unexpected code) additionally surfaces the scoped/unscoped
two-URL rule whenever the configured base URL host is not
api.atlassian.com, and enumerates the three real possibilities: wrong
base URL for the token type, no access to the project, or nonexistent
ticket. Project name derived from the ticket key for accuracy.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: amjithtitus09