Raise ContentValidationFailed for data_validation_error

[loookashow]

A Flux resource write reports a schema violation with
error_code="data_validation_error", which was not mapped. The most common way
to reach a schema violation therefore raised a bare FoxnoseAPIError, and the
documented except ContentValidationFailed never fired for it — callers were
left matching on status_code == 422, which also catches unrelated 422s.

Both codes describe the same failure and carry the same detail payload: the
resource-write path raises RevisionValidationError (data_validation_error),
other paths raise the content variant. Both now map to ContentValidationFailed
and populate errors and errors_truncated identically. error_code still
reports what the server actually sent, so nothing that inspects it changes.

Only these two codes map — an unrelated 422 still falls through to the base
class, which is covered by a test.

Verified against a live backend: a write omitting a required field now raises
ContentValidationFailed with its json_path entries parsed. Mutation-checked
— the new tests fail when the code is dropped from the set.

Found while releasing langchain-foxnose 0.4.0, whose docs currently work around
this by matching on the status code (FoxNoseTech/langchain-foxnose#8). Those can
be simplified once this ships.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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