docs(fetch_url): fix 3 misleading description bits

Three small but high-leverage text corrections. No code or test
changes — these only affect what the LLM sees in tools/list and
Pydantic schemas.

1) FetchUrlRequest.connection_name description
   Old: "The Connection's yarn_rm_url defines the allowed host domain."
   New: explicitly says url_allowlist is the host gate, NOT yarn_rm_url.
        yarn_rm_url is only used by get_external_* tools. Without this
        fix the LLM would try to control fetch scope via yarn_rm_url
        (a no-op) instead of url_allowlist.

2) Connection.url_allowlist description
   Added: "Set or change via save_connection (pass url_allowlist on
   create) or update_connection (PATCH the field on an existing
   connection)." Tells the LLM which tools populate the field,
   instead of leaving it to guess.

3) /fetch_url route description
   Old: "30s timeout, redirects followed."
   New: "30s timeout, redirects followed, response body capped at
        1 MB (the response includes a truncated boolean when this
        kicks in)." LLM previously had no way to discover the 1MB
        cap or the truncated indicator; it would just see
        short responses and assume that was the full body.

Tests: 394 passed, no test changes (descriptions are not asserted).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

spark_executor/server.py

@@ -549,7 +549,8 @@ def _update_job_file(req: UpdateJobFileRequest):
         "guardrails — the allowlist is the only gate — so keep it tight. The "
         "Connection's saved auth is reused, so the agent does not need cluster "
         "credentials.\n\n"
-        "30s timeout, redirects followed."
+        "**Limits:** 30s timeout, redirects followed, response body capped at "
+        "1 MB (the response includes a `truncated` boolean when this kicks in)."
     ),
 )
 def _fetch_url(req: FetchUrlRequest):