docs(fetch_url): drop 1 MB cap mention from route description

I added "response body capped at 1 MB (truncated boolean)" to the
fetch_url route description in 16fe011, not realising the cap was
already removed in deafb5a (refactor: drop fetch_url body cap and
modernize datetime usage). The body cap is gone from both
fetch_url.py (no _MAX_BODY_BYTES, no [:_MAX_BODY_BYTES] slicing, no
truncated field in the result) and from FetchUrlResult (no
truncated field). The route description now falsely advertised a
limit that no longer exists.

Replace the 1 MB / truncated line with a plain "30s timeout,
redirects followed" line that matches the actual code.

Tests: 401 passed, no test changes (description text is not
asserted).

Note (not part of this change): read_job_file and update_job_file
still have a 1 MB cap in tools/job_file.py:27 (MAX_FILE_BYTES =
1 * 1024 * 1024) and the matching description lines. Tell me if
you want those removed too.

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

spark_executor/server.py

@@ -556,8 +556,7 @@ 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"
-        "**Limits:** 30s timeout, redirects followed, response body capped at "
-        "1 MB (the response includes a `truncated` boolean when this kicks in).\n\n"
+        "**Limits:** 30s timeout, redirects followed.\n\n"
         "**Errors:** when the host is unreachable (connect refused, DNS "
         "failure, TLS handshake error, timeout, etc.) this tool returns "
         "HTTP 400 with the exception class and message in the response "