Update CHANGELOG.md

[noahho]

Motivation and Context

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Public API Changes

• No Public API changes
• Yes, Public API changes (Details below)

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How Has This Been Tested?

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Checklist

• The changes have been tested locally.
• Documentation has been updated (if the public API or usage changes).
• A entry has been added to CHANGELOG.md (if relevant for users).
• The code follows the project's style guidelines.
• I have considered the impact of these changes on the public API.

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[Copilot]

Pull Request Overview

This PR updates the CHANGELOG.md file to document recent changes in the Unreleased section, adding entries for new features and bug fixes.

• Added documentation for new finetuned model checkpoints
• Added two bug fix entries for .env file validation and PyTorch compatibility

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[Copilot]

The word 'finetuned' should be hyphenated as 'fine-tuned' for proper spelling.

Suggested change

	- Added several new finetuned model checkpoints. ([#462](https://github.com/PriorLabs/TabPFN/pull/462))
	- Added several new fine-tuned model checkpoints. ([#462](https://github.com/PriorLabs/TabPFN/pull/462))

[gemini-code-assist]

Code Review

This pull request updates CHANGELOG.md with new entries for recent additions and bug fixes under the [Unreleased] section. The new entries are clear and correctly reference their corresponding pull requests. I have added one comment suggesting a minor improvement to standardize section headers for better consistency in the future.

[gemini-code-assist]

The changelog entries are clear. However, I've noticed the heading for bug fixes is inconsistent throughout this file (e.g., ### Bug Fixes, ### Bug fix, ### Fixed). For better consistency and alignment with the Keep a Changelog standard, consider standardizing to ### Fixed in the future. Since this PR follows the existing format for the [Unreleased] section, this is just a suggestion for future improvement and not a blocker.