🐛 Fix String values becoming tuples in latex transformation middlewares

[MiWeiss]

Summary

_PyStringTransformerMiddleware.transform_string assigned the (value, error) tuple returned by _transform_python_value_string directly to string.value, instead of unpacking it like transform_entry does. As a result:

• After LatexDecodingMiddleware/LatexEncodingMiddleware, String.value was a tuple, not a str.
• Transformation errors on @string blocks were silently swallowed (no MiddlewareErrorBlock, unlike for entries).

Fix

transform_string now unpacks the tuple and, on error, returns a MiddlewareErrorBlock wrapping a PartialMiddlewareException — mirroring the transform_entry behavior.

Tests

• En-/decoding a String block yields a str with the transformed value.
• A failing transformation on a String block surfaces as a MiddlewareErrorBlock in library.failed_blocks.

Fixes #529

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