fix: align package metadata with PyPI name

[WH-2099]

Closes #336

Pull Request Checklist

Thank you for your contribution! Before submitting your PR, please make sure you have completed the following checks:

Compatibility Check

• I have checked whether this change affects the backward compatibility of the plugin declared in README.md
• I have checked whether this change affects the forward compatibility of the plugin declared in README.md
• If this change introduces a breaking change, I have discussed it with the project maintainer and specified the release version in the README.md
• I have described the compatibility impact and the corresponding version number in the PR description
• I have checked whether the plugin version is updated in the README.md

Available Checks

• just build has passed
• Relevant documentation has been updated (if necessary)

Summary

• Change the package metadata name from dify_plugin to dify-plugin.
• Keep the Python import package unchanged as dify_plugin.
• Align the package metadata with the release workflow PyPI project URLs and trusted publisher project naming.

Compatibility

This changes only the published distribution metadata name. PyPI normalizes dify_plugin and dify-plugin to the same canonical project name, so the import path and installed package usage remain unchanged.

Validation

• just build passed.
• Verified the generated wheel metadata reports Name: dify-plugin.
• just check was attempted, but the local worktree contains untracked exploratory plugin directories and ruff includes them; the failure is unrelated to this one-file metadata change.

[gemini-code-assist]

Code Review

This pull request updates the project name in pyproject.toml from dify_plugin to dify-plugin to align with PyPI and the CLI binary name. A review comment identifies that the README.md file still references the old name in its dependency examples and should be updated to maintain consistency.