Fix Variables API handling of non-string JSON values

[kaxil]

Fixes #71010
Closes #71015

Problem

VariableBody.value is typed JsonValue, so the Variables REST API accepts any JSON type, but everything downstream assumed a string:

• POST /api/v2/variables with "value": ["a", "b"] returned 201 but stored the Python repr ['a', 'b']. That is not valid JSON, so the variable silently breaks the next Variable.get(key, deserialize_json=True).
• PATCH /api/v2/variables/{key} with the same payload failed with a masked 500: the raw list reached the Fernet encryption step, which raised TypeError: encoding without a string argument.
• Bulk update failed the same way, and one non-string entity took down the whole request. Bulk create already JSON-encoded dicts and lists, but stored booleans and nulls as "True"/"None".

Solution

VariableBody now JSON-encodes non-string values once, at request validation, so POST, PATCH and both bulk actions behave identically: strings are stored verbatim, everything else is stored as JSON (indent=2, byte-identical to the existing bulk-create format) and round-trips through deserialize_json=True. The now-redundant serialize_json special case in bulk create is removed, and Variable.set_val raises a clear TypeError pointing at serialize_json=True instead of the cryptic encoding error.

Request	Before	After
POST "value": ["a", "b"]	201, stores ['a', 'b'], unreadable as JSON	201, stores ["a", "b"]
PATCH "value": ["a", "b"]	500	200
Bulk update with a dict value	Whole request 500	200
POST or bulk "value": true / null	Stores "True" / "None"	Stores true / null
PATCH "value": null	200 but silently kept the old value	200, stores null

String values, including JSON passed as a string, are stored byte-for-byte as before, and the OpenAPI schema is unchanged, so generated clients are unaffected.

Why coerce instead of rejecting with 422

Non-string values are an intentional part of the API contract since #49844: the UI Import Variables flow sends raw parsed JSON (ImportVariablesForm.tsx#L71), the docs describe bulk-uploading variables as a JSON file, and airflow variables import applies the identical encode-iff-not-a-string rule (variable_command.py#L165). Tightening value back to str would 422 all of those.

Notes

• Rows already corrupted on released versions stay as-is; a stored Python repr cannot be safely auto-converted back to JSON.
• Non-finite floats still store as the non-strict-JSON literal NaN. Rejecting them in the validator is not viable: the 422 response echoes the NaN input, which starlette's JSONResponse cannot serialize, turning the rejection into a 500.
• The middleware issue from the report (JWTRefreshMiddleware masking the real traceback as RuntimeError: No response returned) is not addressed here; it also affects When trying to edit and save Airflow variables using the Web UI, the save operation fails with an HTTP 500 Internal Server Error. #68868 and API write endpoints return opaque 500 when the database rejects a payload #66889.