test: add unit tests for pkg/fileutil and pkg/health

[perhapzz]

📝 Description

Add comprehensive unit test coverage for two packages that previously had zero tests:

• pkg/fileutil (9 tests) — atomic file write operations
• pkg/health (15 tests) — health/ready/reload HTTP endpoints

🗣️ Type of Change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 📖 Documentation update
• ⚡ Code refactoring (no functional changes, no api changes)

🤖 AI Code Generation

• 🤖 Fully AI-generated (100% AI, 0% Human)
• 🛠️ Mostly AI-generated (AI draft, Human verified/modified)
• 👨💻 Mostly Human-written (Human lead, AI assisted or none)

🔗 Related Issue

No existing issue — discovered during code review that these two core packages had 0% test coverage.

📚 Technical Context (Skip for Docs)

• Reference URL: N/A
• Reasoning: pkg/fileutil.WriteFileAtomic is used for config persistence on edge devices (SD cards, flash storage) and pkg/health endpoints are critical for Docker/K8s deployments. Both deserve test coverage to prevent regressions.

🧪 Test Environment

• Hardware: Docker container (aarch64)
• OS: Debian 12 (bookworm)
• Model/Provider: N/A (unit tests only)
• Channels: N/A

Test Details

pkg/fileutil — 9 tests:

Test	Description
TestWriteFileAtomic_Basic	Write and read-back verification
TestWriteFileAtomic_Permissions	File permission (0600) enforcement
TestWriteFileAtomic_Overwrite	Atomically overwrite existing file
TestWriteFileAtomic_EmptyData	Handle empty byte slice
TestWriteFileAtomic_CreatesParentDirs	Auto-create nested directories
TestWriteFileAtomic_NoTempFileOnSuccess	No temp file leak after success
TestWriteFileAtomic_LargeFile	1MB file write integrity
TestWriteFileAtomic_Concurrent	Concurrent write safety
TestWriteFileAtomic_InvalidPath	Error on invalid path

pkg/health — 15 tests:

Test	Description
TestHealthHandler_ReturnsOK	/health returns 200 with status, uptime, pid
TestReadyHandler_NotReady	/ready returns 503 when not ready
TestReadyHandler_Ready	/ready returns 200 when ready
TestReadyHandler_FailedCheck	/ready fails when registered check fails
TestReadyHandler_PassingCheck	/ready passes with all checks ok
TestReloadHandler_MethodNotAllowed	/reload rejects non-POST
TestReloadHandler_NoReloadFunc	/reload 503 without handler
TestReloadHandler_Success	/reload calls function, returns 200
TestReloadHandler_Error	/reload returns 500 on function error
TestSetReady_Toggle	Toggle ready state
TestRegisterCheck_MultipleChecks	Multiple check interaction
TestRegisterOnMux	Custom ServeMux registration
TestNewServer	Default initialization
TestStartContext_Cancellation	Graceful shutdown on context cancel
TestStatusString	Helper function

📸 Evidence (Optional)

Click to view test output

=== pkg/fileutil ===
--- PASS: TestWriteFileAtomic_Basic (0.01s)
--- PASS: TestWriteFileAtomic_Permissions (0.00s)
--- PASS: TestWriteFileAtomic_Overwrite (0.00s)
--- PASS: TestWriteFileAtomic_EmptyData (0.00s)
--- PASS: TestWriteFileAtomic_CreatesParentDirs (0.00s)
--- PASS: TestWriteFileAtomic_NoTempFileOnSuccess (0.00s)
--- PASS: TestWriteFileAtomic_LargeFile (0.00s)
--- PASS: TestWriteFileAtomic_Concurrent (0.01s)
--- PASS: TestWriteFileAtomic_InvalidPath (0.00s)
PASS
ok  	github.com/sipeed/picoclaw/pkg/fileutil	0.057s

=== pkg/health ===
--- PASS: TestHealthHandler_ReturnsOK (0.01s)
--- PASS: TestReadyHandler_NotReady (0.00s)
--- PASS: TestReadyHandler_Ready (0.00s)
--- PASS: TestReadyHandler_FailedCheck (0.00s)
--- PASS: TestReadyHandler_PassingCheck (0.00s)
--- PASS: TestReloadHandler_MethodNotAllowed (0.00s)
--- PASS: TestReloadHandler_NoReloadFunc (0.00s)
--- PASS: TestReloadHandler_Success (0.00s)
--- PASS: TestReloadHandler_Error (0.00s)
--- PASS: TestSetReady_Toggle (0.00s)
--- PASS: TestRegisterCheck_MultipleChecks (0.00s)
--- PASS: TestRegisterOnMux (0.00s)
--- PASS: TestNewServer (0.00s)
--- PASS: TestStartContext_Cancellation (0.06s)
--- PASS: TestStatusString (0.00s)
PASS
ok  	github.com/sipeed/picoclaw/pkg/health	0.102s

☑️ Checklist

• My code/docs follow the style of this project.
• I have performed a self-review of my own changes.
• I have updated the documentation accordingly.

[CLAassistant]

All committers have signed the CLA.

[afjcjsbx]

LGTM!