fix: address review findings on fetch-url-tool

- fetch_url: revalidate allowlist on every redirect hop (fixes SSRF where
  302 to disallowed host / 169.254.169.254 / file:// bypassed the
  url_allowlist). Stream response body with iter_bytes and cap at 1MB
  so a multi-GB response from an allowlisted host cannot OOM the service.
  Reuses the manual-redirect-loop pattern from yarn_client.

- list_applications: stop swallowing 404 (YARN returns 200+empty for
  "no match"; 404 means the RM doesn't support the endpoint — surface
  the YarnError instead of hiding it as an empty result). Add
  Field(ge=1, le=10000) to ListApplicationsRequest.limit so a runaway
  limit is rejected at the Pydantic layer with 422.

- save_connection: PATCH semantics for existing records. Re-route to
  update_connection when the name already exists so partial updates
  (e.g. only master) no longer wipe url_allowlist back to []. Uses an
  _UNSET sentinel in the tool function to distinguish "omitted" from
  "None" without breaking the existing parameter list.

- README: drop leading space on 5 new connection-tool table rows that
  was breaking GitHub Flavored Markdown table continuity.

- Indentation: normalize connections.py and requests.py to 4-space
  indent (auth_password/auth_principal/auth_keytab were 3-space).

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

spark_executor/core/yarn_client.py

@@ -299,9 +299,6 @@ def list_applications(
     resp = _request("GET", url, params=params,
                     verify=config.verify_for_httpx(),
                     auth=config.auth_for_httpx())
-    if resp.status_code == 404:
-        # No apps match (or RM doesn't support the endpoint)
-        return []
     if resp.status_code >= 400:
         raise YarnError(
             f"YARN list applications failed: {resp.status_code} {resp.text[:200]}"