chore(core): typed Platform enum + JSON/Text codecs

[miladev95]

Introduce a typed Platform enum with helpers and codecs while preserving legacy constants for compatibility. Add a typed LogBlock with validation.

Changes

• Add Platform (PlatformIOS, PlatformAndroid, PlatformHuawei) with String(), IsValid(), ParsePlatform(string).
• Add MarshalText/UnmarshalText and MarshalJSON/UnmarshalJSON for Platform (supports legacy numeric 1/2/3).
• Add LogBlock with LogSucceededPush, LogFailedPush and IsValid().
• Keep legacy constants PlatFormIos, PlatFormAndroid, PlatFormHuawei, SucceededPush, FailedPush.
• Add focused tests in core/core_test.go.

Backward compatibility

• Existing code using legacy constants continues to work. New typed APIs are opt‑in.

[appleboy]

I will take a look.