fix(prompt-input): report files dropped by a partial rejection

[HardMax71]

When some, but not all, of the added files fail accept or maxFileSize, add now calls onError with that code and a message saying some files were not added, then adds the rest, as maxFiles already does. Both the local and the provider paths change. When every file fails, the behavior and messages stay the same. I updated the onError docs and added a changeset.

Three new tests in __tests__/prompt-input.test.tsx under "partially rejected files": a mixed selection with accept, a mixed selection with maxFileSize, and the accept case inside PromptInputProvider. Each fails on main and passes here. The full @repo/elements suite passes (961 tests).

Closes #511

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