test(uptime): Mock the robots.txt fetch in candidate URL tests (#127947)

Four `ProcessCandidateUrlTest` tests (`test_succeeds_*`) call
`process_candidate_url` for random `https://sentry-<hex>.io` domains.
That fetches `robots.txt` over the real network. The outbound network
guard in report mode (#127944) caught these on a full backend run.

These domains don't resolve, so `check_url_robots_txt` catches the error
and allows the URL. The class now has an autouse `responses` fixture
that answers those `robots.txt` URLs with a 404. `get_robots_txt_parser`
turns that into an empty, allow-all parser, so the tests get the same
outcome without a DNS lookup or a connection. Tests that already patch
`get_robots_txt_parser` don't change. No assertions change.

This PR only changes tests and doesn't depend on #127944. I ran the file
locally with `SENTRY_NETWORK_GUARD=strict`: it passes, and the guard
records no outbound attempts.

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via @joshuarli.
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Co-authored-by: sentry-junior[bot] <264270552+sentry-junior[bot]@users.noreply.github.com>
Co-authored-by: Joshua Li <14209781+joshuarli@users.noreply.github.com>

Author: sentry-junior[bot]

tests/sentry/uptime/autodetect/test_tasks.py

@@ -1,11 +1,15 @@
 from __future__ import annotations
 
+import re
 import uuid
+from collections.abc import Generator
 from datetime import datetime, timedelta
 from unittest import mock
 from unittest.mock import call
 from urllib.robotparser import RobotFileParser
 
+import pytest
+import responses
 from django.utils import timezone
 
 from sentry.deletions.tasks.scheduled import run_scheduled_deletions
@@ -227,6 +231,16 @@ def test_should_not_detect(self) -> None:
 
 @freeze_time()
 class ProcessCandidateUrlTest(UptimeTestCase):
+    @pytest.fixture(autouse=True)
+    def mock_robots_txt_not_found(self) -> Generator[None]:
+        # The generated test domains don't exist, so give them no robots.txt
+        # rather than letting the fetch go out to the network.
+        with responses.RequestsMock(assert_all_requests_are_fired=False) as rsps:
+            rsps.add(
+                responses.GET, re.compile(r"https://sentry-[0-9a-f]{8}\.io/robots\.txt"), status=404
+            )
+            yield
+
     @with_feature("organizations:uptime")
     def test_succeeds_new(self) -> None:
         url = make_unique_test_url()