feat: simplify fetch_url allowlist + add update_connection tool

Part 1 — fetch_url validation simplification
  Now that Connection.allowed_url_hosts exists, it's the ONLY check.
  The old 2-label suffix-overlap rule against yarn_rm_url is gone.

  - Connection.allowed_url_hosts: list[str] = Field(default_factory=list)
    (was list[str] | None = None). Default empty list means the
    connection has no fetch access; the user must explicitly opt in
    via save_connection or update_connection.
  - _validate_url_host drops the yarn_rm_url parameter and the
    _host_suffix_overlap helper. Now: scheme/host/IP checks, then
    empty-reject, then glob match. Reject everything else.
  - fetch_url's error message now points the user at
    Connection.allowed_url_hosts as the fix.
  - The label-by-label glob check from the previous commit is kept
    (SSRF guard: 'ccam*' matches 'ccam50' but NOT 'ccam50.evil.com').

Part 2 — update_connection tool
  PATCH-style update for an existing Connection record. Only the
  fields the caller provides are changed. Same pattern as
  update_pending_job: req.model_dump(exclude_none=True), with
  'name' popped before passing to the store.

  To CLEAR a field (e.g. drop auth_password), use delete_connection
  + save_connection. This is YAGNI; the alternative (model_fields_set
  to distinguish 'omitted' from 'null') adds surface for bugs.

  - ConnectionStore.update(name, **fields): get, model_copy(update=...),
    save. Lock + atomic write. Re-validates the patched record.
  - update_connection tool function: passes fields through to the
    store, logs which fields were changed.
  - UpdateConnectionRequest Pydantic model: 12 mutable fields + name.
  - /update_connection route with operation_id='update_connection'.
  - 9 new unit tests in test_connection_tools.py (PATCH, dict/list
    replace-not-merge, unknown name 404, validation of patched record,
    disk persistence).
  - test_fetch_url.py: existing 21 tests updated; 3 new tests for
    the empty/None/missing allowed_url_hosts cases.
  - test_mcp_routes.py: assert 22 tool routes.
  - README: update_connection row added; fetch_url row updated.

Tests: 386 passed (up from 377, +9 net).

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

spark_executor/models.py

@@ -69,18 +69,18 @@ class Connection(BaseModel):
     auth_principal: str | None = None
     auth_keytab: str | None = None
 
-    allowed_url_hosts: list[str] | None = Field(
-        default=None,
+    allowed_url_hosts: list[str] = Field(
+        default_factory=list,
         description=(
-            "Optional list of fnmatch glob patterns for hosts the fetch_url tool "
-            "may access, in addition to the default 'shares >= 2 labels of suffix "
-            "with yarn_rm_url host' rule. Useful for clusters whose hostnames do "
-            "NOT share a 2+ label suffix — e.g. single-label hosts like 'ccam1'-"
-            "'ccam99' (configure ['ccam*']) or HDFS namenode on a different "
-            "subdomain ('*.hadoop.internal'). Patterns are matched against the "
-            "URL host only (no port, no path). fnmatch rules apply: '*' does NOT "
-            "match '.', so 'ccam*' matches 'ccam50' but not 'ccam50.evil.com'. "
-            "Default None means the suffix rule alone applies."
+            "List of fnmatch glob patterns for hosts the fetch_url tool may access. "
+            "The list is mandatory-opt-in: an empty list (the default) denies all "
+            "hosts, so you must populate it before fetch_url can access any URL. "
+            "Useful for clusters whose hostnames do NOT share a common suffix — "
+            "e.g. single-label hosts like 'ccam1'-'ccam99' (configure ['ccam*']) "
+            "or HDFS namenode on a different subdomain ('*.hadoop.internal'). "
+            "Patterns are matched against the URL host only (no port, no path). "
+            "fnmatch rules apply: '*' does NOT match '.', so 'ccam*' matches "
+            "'ccam50' but not 'ccam50.evil.com'."
         ),
     )