Add optional exception for failed tasks in wait_for_task

[chenyujia1024]

Related issue

Related to #747

What does this PR do?

Adds an opt-in raise_on_failure option to wait_for_task.

• Existing behavior remains unchanged by default.
• When raise_on_failure=True, a task with status failed raises MeilisearchTaskFailedError.
• Successful, canceled, and other terminal tasks continue to return Task.
• Existing polling timeout behavior remains unchanged.

raise_on_failure is keyword-only and defaults to False, so existing calls such as:

client.wait_for_task(uid)
client.wait_for_task(uid, 5000, 50)

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