Migrate linkerd + istio v1 tests off requests_mock

[mwdd146980]

Motivation

Step 6 PR 6.1 of the httpx test-decoupling migration. Two integrations still mock at the requests layer via requests_mock.Mocker(), which couples test mocks to the underlying HTTP library and blocks the eventual requests → httpx swap.

Plan: https://datadoghq.atlassian.net/wiki/spaces/AI/pages/6692406054

Approach

Replace requests_mock.Mocker() with mock_openmetrics_http (the OpenMetrics v1 mixin fixture that patches OpenMetricsScraperMixin.get_http_handler) and MockHTTPResponse. v2 tests already use mock_http_response and are unchanged.

Pattern A (text response):

def test_X(aggregator, dd_run_check, mock_openmetrics_http):
    mock_openmetrics_http.get.return_value = MockHTTPResponse(
        content=get_response(...), headers={'Content-Type': 'text/plain'},
    )
    ...

Pattern B (exception): set mock_openmetrics_http.get.side_effect.

7 tests across 2 files: linkerd/tests/test_linkerd.py (3 v1 tests), istio/tests/test_unit_istio_v1.py (4 tests).

Verification

• ddev test -fs linkerd istio — clean.
• ddev --no-interactive test linkerd — 4 passed, 1 e2e skipped.
• ddev --no-interactive test istio — 24 passed, 2 e2e skipped.

Stacked on mwdd146980/httpx-migration-base.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.12%. Comparing base (20e464b) to head (ce89c14).

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