fix: interpolate actual values in Message.__repr__ f-string

[Aokizy2]

Rationale for this change

Message.__repr__ showed format placeholders ({self.type}) instead of actual values.

What changes are included in this PR?

Add missing f prefix to f-string in Message.__repr__ (pyarrow/lib.pxi).

Are these changes tested?

Yes: added test verifying string representation shows actual values.

Are there any user-facing changes?

Yes (bug fix): str(message) now shows values like type: RecordBatch instead of placeholders.
related to issue #48608

[github-actions]

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