Fix race condition

[dmthong181096]

This PR resolves the race conditions and data races reported in #1 when multiple tasks are run concurrently and their states/metadata are accessed or updated.

Changes:

• Task Field Encapsulation: Unexported all mutable fields of Task (state, err, metadata, history, startedAt, finishedAt) to prevent direct, unsafe concurrent reads/writes from other goroutines.
• Thread-Safe Accessors: Added synchronized getters/setters (using sync.RWMutex) for all encapsulated fields.
• Atomic Transitions: Created helper methods (Start(), Complete(), and Fail(err)) to guarantee atomic transitions of task states, start/finish times, and history updates.
• Runner Refactoring: Updated Runner.Run() to use these thread-safe helper methods instead of locking and modifying fields inline.

Verified that all tests compile and pass successfully, including parallel execution checks.

[dmthong181096]

/claim #1

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