fix: add structured help JSON and provider BASE_URL validation

#683-#692: help topic JSON now includes usage, purpose, formats,
related, local_only, requires_credentials, and aliases extracted
from help prose. Export and doctor keep their custom structured
responses.

#466: new check_base_url_health() validates ANTHROPIC_BASE_URL,
OPENAI_BASE_URL, XAI_BASE_URL, and DASHSCOPE_BASE_URL for basic
HTTP(S) URL format. Non-http schemes and empty values produce a
warn-level diagnostic.

Generated with https://github.com/Yeachan-Heo/gajae-code
Co-authored-by: Gajae Code <dev@gajae-code.com>

rust/crates/rusty-claude-cli/tests/output_format_contract.rs

@@ -1476,7 +1476,7 @@ fn doctor_and_resume_status_emit_json_when_requested() {
         .is_some_and(|available| available.iter().any(|name| name == "web_fetch")));
 
     let checks = doctor["checks"].as_array().expect("doctor checks");
-    assert_eq!(checks.len(), 11);
+    assert_eq!(checks.len(), 12);
     let check_names = checks
         .iter()
         .map(|check| {
@@ -1496,6 +1496,7 @@ fn doctor_and_resume_status_emit_json_when_requested() {
         check_names,
         vec![
             "auth",
+            "base urls",
             "config",
             "mcp validation",
             "hook validation",