Add unit tests for internal/devlake (client, state, discovery)

[ewega]

Problem

internal/devlake is the core package — every CLI command depends on client.go (API calls), state.go (state file persistence), and discovery.go (instance auto-detection). Yet go test ./... reports [no test files] for this package.

The riskiest untested code is SaveState() (state.go:122-149), which reads existing JSON from disk, overlays new state fields, and writes back. A bug in the merge logic could silently corrupt state files, losing deployment metadata, connection IDs, or project references.

With 10 new plugins landing in v0.4.0, each adding scope types and connection entries, an undetected regression in the API client or state management would cascade across the entire CLI.

Scope of Changes

client_test.go

• Use httptest.NewServer to mock DevLake API responses
• Test doGet[T], doPost[T], doPut[T], doPatch[T] with success/error/malformed JSON
• Test ListConnections, CreateConnection, DeleteConnection
• Test PutScopes, ListScopes
• Test CreateProject, PatchBlueprint, TriggerBlueprint
• Test Ping, Health

state_test.go

• Roundtrip: SaveState → LoadState preserves all fields
• Merge behavior: SaveState overlays onto existing Azure metadata without clobbering
• FindStateFile priority: matching endpoint → first existing → create new
• UpdateConnections appends/replaces correctly
• Edge cases: empty file, invalid JSON, missing file

discovery_test.go

• inferLocalCompanionURLs returns correct Grafana/ConfigUI for known ports
• Discover with explicit URL, state file, localhost fallback
• tryStateFile with unreachable backend

Acceptance Criteria

• go test ./internal/devlake/... passes
• Coverage for SaveState merge logic (the highest-risk code path)
• Coverage for all generic HTTP helpers (doGet, doPost, doPut, doPatch)
• Coverage for Discover fallback chain
• go build ./... and go vet ./... pass