feat: add fetch_url tool for proxying HTTP GET to cluster-internal URLs

Add a new MCP tool that lets the agent fetch URLs on the cluster's
network (YARN tracking UI, Spark History Server, NodeManager web UIs)
when the agent is on a different network and cannot reach those hosts
directly.

The MCP service runs on the YARN RM node, so it can reach every host
the cluster knows about — the agent just needs a way to ask.

Security: SSRF guard via host suffix overlap
  - URL host must share >= 2 labels of suffix with the named
    Connection's yarn_rm_url host (e.g. yarn_rm_url='rm.prod.internal'
    allows 'http://nm01.prod.internal/...')
  - IP literals (10.0.0.1, ::1) rejected
  - Non-http(s) schemes (file://, gopher://, ftp://) rejected
  - Connection with no yarn_rm_url cannot use this tool
- Reuses Connection.auth_for_httpx() and verify_for_httpx() so the
  agent does not need cluster credentials
- Response body capped at 1 MB (truncated=true if larger)
- 30s timeout, follows redirects, loguru INFO audit log on every call

- spark_executor/tools/fetch_url.py: new tool + 2 helpers
  (_host_suffix_overlap, _validate_url_host)
- spark_executor/models.py: FetchUrlResult Pydantic model
- spark_executor/tools/requests.py: FetchUrlRequest with descriptions
- spark_executor/server.py: /fetch_url route, operation_id='fetch_url'
- tests/unit/test_fetch_url.py: 13 unit tests covering all guards,
  truncation, auth/SSL pass-through, redirect follow
- tests/integration/test_mcp_routes.py: assert 21 tool routes
- README.md: 1 row in Spark Executor 工具 table

Tests: 369 passed (up from 356).

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

spark_executor/tools/fetch_url.py

@@ -0,0 +1,108 @@
+# coding=utf-8
+"""
+@Time   :2026/7/9
+@Author :tao.chen
+
+Generic HTTP GET proxy for the agent. Lets the agent fetch URLs on the
+cluster's network when it cannot reach those hosts directly. Security:
+URL host must share >= 2 labels of suffix with the connection's yarn_rm_url
+host; IP literals and non-HTTP schemes are rejected. Reuses the connection's
+saved auth/SSL config so the agent doesn't need cluster credentials.
+"""
+import ipaddress
+from urllib.parse import urlparse
+
+import httpx
+
+from common.logging import logger
+from spark_executor.core.yarn_client import YarnClientConfig
+from spark_executor.models import FetchUrlResult
+from spark_executor.tools.connections import store as conn_store
+
+_MAX_BODY_BYTES = 1_000_000  # 1 MB cap on response body
+_REQUEST_TIMEOUT_SECONDS = 30
+
+
+def _host_suffix_overlap(host_a: str, host_b: str, min_labels: int = 2) -> bool:
+    """Return True if host_a and host_b share at least min_labels suffix labels."""
+    labels_a = host_a.lower().split(".")
+    labels_b = host_b.lower().split(".")
+    n = 0
+    i, j = len(labels_a) - 1, len(labels_b) - 1
+    while i >= 0 and j >= 0 and labels_a[i] == labels_b[j]:
+        n += 1
+        i -= 1
+        j -= 1
+    return n >= min_labels
+
+
+def _validate_url_host(url: str, yarn_rm_url: str | None) -> None:
+    """Validate that url is an allowed http(s) hostname on the cluster network."""
+    parsed = urlparse(url)
+    if parsed.scheme not in ("http", "https"):
+        raise ValueError(f"URL scheme must be http or https, got {parsed.scheme!r}")
+
+    host = parsed.hostname
+    if not host:
+        raise ValueError(f"URL has no host: {url!r}")
+
+    try:
+        ipaddress.ip_address(host)
+    except ValueError:
+        pass
+    else:
+        raise ValueError(
+            f"URL host {host!r} is an IP literal — IP targets are not allowed. "
+            "Use a hostname on the cluster network."
+        )
+
+    if not yarn_rm_url:
+        raise ValueError(
+            "Connection has no yarn_rm_url set, cannot validate URL host. "
+            "Save a Connection with yarn_rm_url first."
+        )
+
+    anchor = urlparse(yarn_rm_url).hostname or ""
+    if not _host_suffix_overlap(host, anchor, min_labels=2):
+        raise ValueError(
+            f"URL host {host!r} does not share enough suffix with the connection's "
+            f"yarn_rm_url host {anchor!r} (need >= 2 labels of common suffix). "
+            "Reject this fetch to prevent SSRF."
+        )
+
+
+def fetch_url(url: str, connection_name: str) -> FetchUrlResult:
+    """Proxy an HTTP GET to url using the auth/SSL settings of connection_name."""
+    logger.debug(f"fetch_url enter url={url} connection_name={connection_name}")
+    conn = conn_store.get(connection_name)
+    if conn is None:
+        raise KeyError(f"Connection not found: {connection_name}")
+
+    _validate_url_host(url, conn.yarn_rm_url)
+
+    config = YarnClientConfig.from_connection(conn)
+    resp = httpx.get(
+        url,
+        auth=config.auth_for_httpx(),
+        verify=config.verify_for_httpx(),
+        timeout=_REQUEST_TIMEOUT_SECONDS,
+        follow_redirects=True,
+    )
+
+    body = resp.text[:_MAX_BODY_BYTES]
+    truncated = len(resp.text) > _MAX_BODY_BYTES
+
+    logger.info(
+        f"fetch_url ok url={url} connection_name={connection_name} "
+        f"status_code={resp.status_code} "
+        f"content_type={resp.headers.get('content-type', '')} "
+        f"body_bytes={len(body)} truncated={truncated}"
+    )
+
+    return FetchUrlResult(
+        url=url,
+        status_code=resp.status_code,
+        content_type=resp.headers.get("content-type", ""),
+        body=body,
+        truncated=truncated,
+    )