test: bound vault test-server startup so an unreachable VAULT_ADDR can't hang the suite

[Moep90]

What

The vault test-server helper (tests/vault_server.py) polled the server in a while not initialized: loop with no timeout and except: continue. If the vault it talks to is unreachable, that loop never exits.

When VAULT_ADDR points at a vault that doesn't resolve/respond (common locally — a stale env var, VPN off, no Docker), the whole test suite hangs forever instead of failing. It bites in test_cli.py, test_vault.py and test_vault_transit.py, whose setUpClass builds the shared vault server.

Why

I hit this running the full suite locally: it stalled on test_cli.py::CliFuncsTest::test_cli_inventory for 20+ minutes. The process wasn't deadlocked — it was retrying a health check against an unreachable VAULT_ADDR in an unbounded loop, logging status is {} / NameResolutionError endlessly. CI never sees it because vault is reachable there.

Approach

• vault_server.py: bound both startup loops (container-running wait and the health-check/init wait) with a deadline (KAPITAN_VAULT_SETUP_TIMEOUT, default 60s). On timeout raise VaultServerError instead of looping. Cache construction failures so each dependent test class skips instantly rather than re-waiting the full timeout.
• test_cli / test_vault / test_vault_transit: catch VaultServerError in setUpClass and SkipTest — the same graceful-skip behaviour the reclass-rs tests already use when their backend is unavailable. Teardown guarded for the skipped case.
• New tests/test_vault_server.py: regression test asserting that an unreachable VAULT_ADDR makes VaultServer() raise quickly instead of hanging.

Verification

• Regression test: red→green (infinite hang → raises in ~4s).
• Full non-slow suite on this branch with an unreachable VAULT_ADDR: 522 passed, 36 skipped in ~33s (previously hung indefinitely). The 35 vault tests skip with a clear reason.
• ruff check / ruff format clean; pre-commit hooks pass.

CI is unaffected: vault is reachable there, so the loops exit immediately and nothing skips — behaviour is unchanged on CI.

[Moep90]

Relates to #1512 — this is the test-isolation/hang half of that test-infra issue (bounding the vault test-server startup).