fix: resolve create_model() TypeError in Partial class by pinning pydantic version to ^2.8.0

[vikyw89]

Reference: https://github.com/jxnl/instructor/pull/801

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:rocket:	This description was created by Ellipsis for commit 42f08b7d6895b0243b1fab90680ba5c1d51e03a8

Summary:

Update pydantic version to ^2.8.0 in pyproject.toml to resolve TypeError in Partial class's create_model() function.

Key points:

• Update pydantic version in pyproject.toml from ^2.7.0 to ^2.8.0
• Resolves TypeError in Partial class's create_model() function

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