test: baseline pure-logic tests across manager + agent

[kleopasevan]

Summary

Adds 70 pure-logic tests across modules the upcoming pluggable-VMM
backend refactor will touch. Captures current behavior so refactors
can detect regressions.

Test count: 25 → 95 across the workspace (no integration tests; all
pure logic, no DB, no HTTP, no real VMM).

Per-module breakdown

Module	Tests	Notes
manager/vms/service.rs	14	network selection, rate-limiter normalization, VmPaths, tap-name format, snapshot payloads, URL query encoding
manager/snapshots/{routes,repo}.rs	18	URL building, payload shapes, name resolution, mem-path handling. 7 pure helpers extracted from inlined async handlers for testability
manager/templates/{routes,repo}.rs	7	TemplateSpec round-trip, TemplateRow JSON decoding
manager/hosts/routes.rs	8	status thresholds, metrics extraction, row mapping. 3 helpers + 2 threshold constants extracted
manager/networks/{service,routes}.rs	8	gateway derivation, DHCP range, host-IP parsing, list-item mapping
agent/vm/spawn.rs	5	unit-name format, SpawnReq serde, socket connectability
agent/vm/{stop,serial,balloon,entropy,system}.rs	8	request serde round-trips, config-path layouts

Production code refactors (in scope, byte-for-byte equivalent)

• snapshots/routes.rs — extracted 7 pure helpers (build_agent_snapshot_urls, resolve_snapshot_name, resolve_snapshot_type, build_create_snapshot_payload, combined_snapshot_size_i64, resolve_storage_mem_path, resolve_instantiate_name) and a AgentSnapshotUrls struct. Original behavior preserved including Some("") override semantics and None mem path serializing as JSON null.
• hosts/routes.rs — extracted compute_host_status, extract_host_metrics, host_row_to_list_item plus HEALTHY_THRESHOLD_SECONDS = 30 and DEGRADED_THRESHOLD_SECONDS = 300 constants. Call sites in heartbeat/list/get rewired to call helpers.

No other production code modified. No new dependencies.

Verification

• cargo test --workspace --no-fail-fast → 72 manager + 23 agent passing, 11 ignored (pre-existing)
• cargo clippy --all-targets --all-features -- -D warnings → clean
• cargo fmt -- --check → clean

What this enables

The companion VMM-backend spec (docs/superpowers/specs/2026-04-28-pluggable-vmm-backend.md, separate PR) refactors apps/agent/src/features/vm/spawn.rs, snapshot routes, and the manager's spawn_firecracker flow into a VmmDriver trait. With this baseline in place, that refactor can be verified to preserve current behavior — without it, "no regression" is vacuous (only 25 tests existed beforehand, mostly leaf helpers).

Test plan

• cargo test --workspace
• cargo clippy --all-targets --all-features -- -D warnings
• cargo fmt -- --check
• Reviewer spot-check: extracted helpers in snapshots/routes.rs and hosts/routes.rs preserve original semantics

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