fix(ci): validate-tests-merge exits 1 on zero merged test files too

[JSONbored]

Summary

• fix(ci): don't fail scoped test selection when zero test files match #8167 fixed the per-shard --changed case (exits 0 with "No test files found"), but --mergeReports exits 1 for the same condition -- a different code path, not covered by that fix.
• Confirmed live: docs(discovery-plane): finalize operator guide now that the plane is shipped and live #8168 (the same docs-only PR fix(ci): don't fail scoped test selection when zero test files match #8167 was meant to unblock) passed all 3 validate-tests shards but still failed validate-tests-merge with "No test files found, exiting with code 1", even with COVERAGE_NO_THRESHOLDS already disabling the threshold check.
• Only treats that exact case as success, and only when COVERAGE_NO_THRESHOLDS is set (i.e. this run is the scoped case to begin with) -- in the unscoped full-suite case, "no test files found" while merging is still a real failure worth surfacing loudly.

Test plan

• Workflow-only change to .github/workflows/ci.yml; no src/** changes, so no Codecov impact.
• actionlint .github/workflows/ci.yml passes.
• docs(discovery-plane): finalize operator guide now that the plane is shipped and live #8168 going green on validate-tests-merge once this merges and it's re-triggered is the real verification.

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
20673	1	20672	17

View the top 1 failed test(s) by shortest run time

test/unit/codecov-policy.test.ts > Codecov policy > fails closed when the backend coverage report is missing or cannot upload

Stack Traces | 0.129s run time

AssertionError: expected '${{ success() && steps.coverage.outpu…' to be '${{ success() }}' // Object.is equality

Expected: "${{ success() }}"
Received: "${{ success() && steps.coverage.outputs.no_tests_matched != 'true' }}"

 ❯ test/unit/codecov-policy.test.ts:62:35

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