feat(LinuxTentacleE2E): Phase 12.M.L.C.2 — register failure paths (401 + incomplete payload)

Summary

Two surgical failure-mode pins in one PR. Both reuse J.M.L.C.1's LinuxStubSquidServer + RegisterTestContext; only differ in stub configuration.

C1.u1 — 401 unauthorized → exit non-zero WITHOUT retry

Real-world driver: operator typos --api-key OR key revoked server-side.

Production contract: 4xx errors are NOT retried (TentacleRegistrationClient.RegisterAsync line 56-60).

Without this pin, regressions ship silently:

• 4xx retry regression → operator hangs ~5min on each typo'd API key (10 retries × exponential backoff)
• Exception swallowing → operator sees "success" but no config persisted

C1.u4 — incomplete payload → exit non-zero

Real-world driver: server-side regression / proxy injection returns malformed JSON (machineId=0 OR empty serverThumbprint).

Production contract: EnsureRegistrationPayloadComplete (line 174-177) rejects.

Without this pin, regressions ship silently:

• Operator sees "success" with bogus identity
• Server can't dispatch to the registered agent
• Subsequent dispatches reject with "machine not found"

Assertions (both tests)

Assertion	Pins
exitCode != 0	Failure path actually rejects
C1.u1: stub received exactly 1 call	4xx no-retry contract
C1.u4: stub received call	Validation runs after payload received
Config file NOT at /etc/squid-tentacle/instances/Default.config.json	Failure-path doesn't persist stale config

Skipped from this PR

C1.u3 (5xx retry behavior): production retries 5xx with exponential backoff (1s → 60s × 10 attempts ≈ 5min). Testing requires overriding TentacleRegistrationClientOptions — needs a test-only escape hatch. Defer to dedicated PR with that infrastructure.

Fidelity tier

🟢 High (Rule 12.4): real binary + real HTTP. Same fixture as C1.h; differs only in stub config (status code + body override).

Expected runtime: ~2s combined.

Test plan

• Linux E2E workflow runs (manual workflow_dispatch after merge)
• C1u1_RegisterListening_Server401_ExitsNonZeroWithoutRetry passes within ~3s
• C1u4_RegisterListening_IncompletePayload_ExitsNonZero passes within ~3s
• No regression on existing 33 Linux E2E tests

🤖 Generated with Claude Code