fix(tests): filter PostHog telemetry from fetch mocks

[la14-1]

Why: Two tests fail consistently in the full suite (0% pass rate) due to PostHog telemetry fetch calls leaking into globalThis.fetch mocks, corrupting callCount-based response routing.

Summary

• hetzner-cov.test.ts: "cleans up orphaned primary IPs" test fails because PostHog calls shift mock response order (HTTP 403 returned at wrong time)
• digitalocean-token.test.ts: "attempts OAuth recovery on 401" test gets callCount=4 instead of 2 (2 extra PostHog calls)
• Root cause: telemetry.test.ts enables the PostHog singleton which persists across test files in the same bun process
• Fix: filter non-test URLs in affected fetch mocks so telemetry calls get a benign response without incrementing callCount

Test plan

• Full suite: 2108 pass, 0 fail (was 2106 pass, 2 fail)
• Both affected test files pass in isolation
• Biome lint clean

-- spawn-refactor/test-engineer

[la14-1]

Note: This PR overlaps with #3341 — both fix the same 2 flaky tests (hetzner-cov.test.ts and digitalocean-token.test.ts) by addressing fetch mock pollution from concurrent test files. #3341 also includes cmdrun-happy-path.test.ts fixes and uses URL-pattern-aware mock routing (more robust). Recommend merging #3341 first; this PR may then be superseded.

-- refactor/pr-maintainer

[la14-1]

Supersession note: This PR (#3353) is likely superseded by #3358 (root cause fix) + #3365 (more robust defense in depth). If both of those land, this PR can be closed.

Leaving open for now pending human decision on merge order.

-- spawn-refactor/pr-maintainer

[la14-1]

Closing as superseded by #3376, which provides a more comprehensive fix for the same telemetry singleton leak (runtime env-var guard in sendEvent() + SPAWN_TELEMETRY=0 in test preload).

-- refactor/pr-maintainer

[la14-1]

Superseded by #3376.