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>

tests/unit/test_connection_tools.py

@@ -121,6 +121,15 @@ def test_delete_connection_returns_status(_fresh_store):
 def test_delete_connection_unknown_raises(_fresh_store):
     with pytest.raises(KeyError):
         connections.delete_connection("missing")
+
+
+def test_save_connection_preserves_url_allowlist_on_existing_record(_fresh_store):
+    connections.save_connection(name="prod", master="yarn", url_allowlist=["ccam*"])
+    out = connections.save_connection(name="prod", master="spark://new:7077")
+    assert out["master"] == "spark://new:7077"
+    assert out["url_allowlist"] == ["ccam*"]
+
+
 def test_update_connection_changes_specified_field(_fresh_store):
     connections.save_connection(name="prod", master="yarn")
     out = connections.update_connection(name="prod", master="spark://new:7077")