feat(fetch_url): per-Connection allowed_url_hosts glob allowlist

The default 'URL host shares >= 2 labels of suffix with yarn_rm_url
host' rule is too strict for clusters whose hostnames are single-label
(e.g. 'ccam1' through 'ccam99'). The user's cluster is reachable at
http://ccam1:8088, and they want fetch_url to work for any ccamN
host — but ccam1 and ccam50 share 0 suffix labels, so the existing
rule rejects everything.

Add an opt-in allowlist field on Connection:

  allowed_url_hosts: list[str] | None

Each entry is an fnmatch glob pattern. The URL host is allowed if it
matches ANY pattern, regardless of the suffix rule. Save a Connection
with ['ccam*'] to allow ccam1, ccam2, ..., ccam99.

SSRF safety: the implementation does NOT use vanilla fnmatch on the
flat host string (because '*' in fnmatch crosses '.', so 'ccam*' would
match 'ccam50.evil.com' — a security hole). Instead, both the pattern
and the host are split on '.' and matched LABEL-BY-LABEL with the
label counts required to match exactly. So 'ccam*' matches 'ccam50'
but NOT 'ccam50.evil.com' (different label counts).

Test coverage:
- 8 new tests in tests/unit/test_fetch_url.py (glob allow, glob deny,
  dot-boundary SSRF test, multiple globs, empty list, None fallback,
  error message hint)
- All 21 fetch_url tests pass; 377 total.

- spark_executor/models.py: Connection.allowed_url_hosts (with description)
- spark_executor/tools/requests.py: SaveConnectionRequest.allowed_url_hosts
- spark_executor/tools/fetch_url.py: new _host_matches_any_glob helper;
  _validate_url_host now takes allowed_hosts and checks globs BEFORE
  the suffix rule
- tests/unit/test_fetch_url.py: 8 new tests
- README.md: fetch_url row mentions the allowlist

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

README.md

@@ -120,7 +120,7 @@ MCP 客户端需要先执行 `initialize` 握手，拿到 `mcp-session-id` 后
 | `get_external_job_status` | 查询**非本服务提交**的外部 YARN application 状态（按 `application_id` + `connection_name`） |
 | `get_external_job_result` | 查询外部 YARN application 终态结果视图 |
 | `get_external_job_logs` | 拉取外部 YARN application 的聚合日志 |
-| `fetch_url` | 代理 HTTP GET 到集群内网 URL (受 `Connection.yarn_rm_url` host 围栏约束) |
+| `fetch_url` | 代理 HTTP GET 到集群内网 URL (host 围栏: 默认 2 段后缀匹配 `yarn_rm_url`, 或匹配 `Connection.allowed_url_hosts` glob) |
 
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