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/tools/requests.py

@@ -117,10 +117,10 @@ class SaveConnectionRequest(BaseModel):
         description=(
             "Absolute path to a Kerberos keytab file. Optional convenience "
             "for 'kinit -kt' workflows. The service does NOT auto-initialize "
-           "from the keytab — you must `kinit -kt <auth_keytab> <auth_principal>` "
-           "yourself before calling the tools."
-      ),
-  )
+            "from the keytab — you must `kinit -kt <auth_keytab> <auth_principal>` "
+            "yourself before calling the tools."
+        ),
+    )
 
     url_allowlist: list[str] | None = Field(
         default=None,
@@ -504,6 +504,8 @@ class ListApplicationsRequest(BaseModel):
     )
     limit: int = Field(
         default=100,
+        ge=1,
+        le=10000,
         description=(
             "Maximum number of applications to return. YARN has no "
             "offset-based pagination, so for large clusters use state/queue "