feat: add cloudflare_healthcheck v4 to v5 migration

Implements complete migration for cloudflare_healthcheck resource from v4 to v5.

**Major Changes:**
- Pattern 0 migration: Flat structure → Nested attributes (http_config/tcp_config)
- HTTP/HTTPS fields moved into http_config nested attribute
- TCP fields moved into tcp_config nested attribute
- Headers transformed from Set to Map structure
- Numeric fields converted from Int to Float64

**Implementation:**

1. **Helper Functions** (internal/transform/hcl/attributes.go):
   - CreateNestedAttributeFromFields(): Creates nested attributes from field maps
   - MoveAttributesToNestedObject(): Moves multiple attributes into nested objects

2. **Migration Logic** (internal/resources/healthcheck/v4_to_v5.go):
   - TransformConfig(): Handles HCL configuration transformations
   - TransformState(): Handles JSON state transformations
   - transformToHTTPConfig(): Creates http_config for HTTP/HTTPS types
   - transformToTCPConfig(): Creates tcp_config for TCP type
   - buildHeaderMapTokens(): Converts header blocks to map format

3. **Tests** (internal/resources/healthcheck/):
   - v4_to_v5_test.go: 19 unit tests (70.3% coverage)
   - integration_state_test.go: 3 integration tests (30 instances)

4. **Integration Test Data** (integration/v4_to_v5/testdata/healthcheck/):
   - input/: 28 v4 resource configurations (525 lines)
   - expected/: Generated v5 output for validation

5. **CLI Registration** (cmd/tf-migrate/main.go):
   - Import healthcheck migrator for automatic registration

**Test Results:**
- ✅ All 19 unit tests passing
- ✅ All 30 integration instances transforming successfully
- ✅ CLI tool successfully migrates both config and state files
- ✅ 100% success rate on test data

**Validation:**
- State transformation: Production ready
- Config transformation: Complete and working
- Complex transformations: Headers, nested attributes, type conversions all working

Co-Authored-By: Claude <noreply@anthropic.com>

Author: 2 people

cmd/tf-migrate/main.go

@@ -18,6 +18,9 @@ import (
 	"github.com/cloudflare/tf-migrate/internal/pipeline"
 	"github.com/cloudflare/tf-migrate/internal/registry"
 	"github.com/cloudflare/tf-migrate/internal/transform"
+
+	// Import resource migrators so their init() functions run
+	_ "github.com/cloudflare/tf-migrate/internal/resources/healthcheck"
 )
 
 type config struct {