[v3-3-test] Stop variables export/import from silently corrupting values (#70944)

[github-actions]

• Stop variables export/import from silently corrupting values

airflow variables export writes a bare value for any variable without a
description, while airflow variables import treats every dict carrying a
"value" key as a {"value": ..., "description": ...} envelope. A variable whose
own value happens to have that shape is therefore unwrapped on the way back in:
its value is replaced by the inner "value" and a description is invented from
the inner "description". Nothing warns the operator, so a round-trip through a
file - the documented way to migrate variables between environments - quietly
rewrites their data.

Export also decodes the stored JSON before writing it out, which erases the
difference between a variable holding raw text and one holding a JSON-encoded
string. Re-importing flattens the latter, and any Dag reading it with
deserialize_json=True starts failing on a value that is no longer valid JSON.

Import's reconstruction is deterministic - strings are stored verbatim,
everything else is JSON-encoded, and one envelope layer is unwrapped - so export
alone can be made lossless. Fixing it there leaves import untouched and keeps
hand-written import files working exactly as before.

• apply suggestion

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(cherry picked from commit f549b34)

Co-authored-by: Y-C easoneason0905@gmail.com
Co-authored-by: Eason09053360 185830721+Eason09053360@users.noreply.github.com