Validate the request body on a finding metadata PUT

[svader0]

Description

_edit_metadata handles the PUT on /api/v2/findings/{id}/metadata/. It declares FindingMetaSerializer in its schema. It never builds that serializer. It reads name and value off request.data and writes them straight into update_or_create.

Two things go wrong:

• If a client sends a value longer than 300 characters, Postgres raises DataError. The except IntegrityError below does not catch DataError. The caller gets a 500.
• If a client omits name, the code writes NULL. The caller gets a 400, but the message says "Update failed because the new name already exists". That message is wrong.

_add_metadata sits directly below and handles the POST. It already validates the same body with the same serializer. This change makes the PUT branch do the same.

One other change: the 200 response now returns the serialized name and value. Before, it echoed the raw request body.

Test results

I added two tests to FindingMetadataTest in unittests/test_rest_framework.py. I ran the class both ways.

	Result
With the fix	Ran 22 tests ... OK (skipped=10)
Without the fix	FAILED (failures=2, skipped=10)

The unfixed run shows the bug:

django.db.utils.DataError: value too long for type character varying(300)
AssertionError: 400 != 500

Ruff 0.16.0 passes.

Documentation

No documentation change. The endpoint contract stays the same. The declared schema already promised this validation.

Checklist

• Bugfix, submitted against bugfix.
• Code is Ruff compliant.
• Tests added to the unit tests.
• No model change, so no migration.
• No new settings.