LaunchDarkly import accepts empty LD token

[sentry]

Sentry Issue: FLAGSMITH-API-68H

BadSignature: No ":" found in value
(2 additional frame(s) were not displayed)
...
  File "integrations/launch_darkly/tasks.py", line 24, in process_launch_darkly_import_request
    process_import_request(import_request)
  File "integrations/launch_darkly/services.py", line 1110, in process_import_request
    ld_token = _unsign_ld_value(
  File "integrations/launch_darkly/services.py", line 56, in _unsign_ld_value
    return signing.loads(  # type: ignore[no-any-return]

{'import_request_id': 609, 'ld_project_key': 'browser-extension', 'organisation_id': 32015, 'project_id': 42860, 'event': 'import-failed', 'logger': 'launch_darkly', 'level': 'error', 'exception': 'Traceback (most recent call last):\n  File "/app/integrations/launch_darkly/tasks.py", line 24, in process_launch_darkly_import_request\n    process_import_request(import_request)\n    ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^\n  File "/app/integrations/launch_darkly/services.py", line 1110, in process_import_request\n    ld_token = _unsign_ld_value(\n        import_request.ld_token,\n        import_request.created_by.id,\n    )\n  File "/app/integrations/launch_darkly/services.py", line 56, in _unsign_ld_value\n    return signing.loads(  # type: ignore[no-any-return]\n           ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n        value,\n        ^^^^^^\n        salt=f"ld_import_{user_id}",\n        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n    )\n    ^\n  File "/build/.venv/lib/python3.13/site-packages/django/core/signing.py", line 194, in loads\n    ).unsign_object(\n      ~~~~~~~~~~~~~^\n        s,\n        ^^\n        serializer=serializer,\n        ^^^^^^^^^^^^^^^^^^^^^^\n        max_age=max_age,\n        ^^^^^^^^^^^^^^^^\n    )\n    ^\n  File "/build/.venv/lib/python3.13/site-packages/django/core/signing.py", line 267, in unsign_object\n    base64d = self.unsign(signed_obj, **kwargs).encode()\n              ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^\n  File "/build/.venv/lib/python3.13/site-packages/django/core/signing.py", line 291, in unsign\n    result = super().unsign(value)\n  File "/build/.venv/lib/python3.13/site-packages/django/core/signing.py", line 232, in unsign\n    raise BadSignature(\'No "%s" found in value\' % self.sep)\ndjango.core.signing.BadSignature: No ":" found in value', 'timestamp': '2026-07-06T14:07:26.994237Z'}

Acceptance criteria

• The API responds with 400 if token is empty.

[zain-asif-dev]

Opened a fix here: #7954

Worth noting: the create endpoint already rejects a blank token with a 400 today (CreateLaunchDarklyImportRequestSerializer.token is a CharField with the default allow_blank=False), verified this locally against main. The actual crash in the Sentry trace happens later, in process_import_request: once an import request finishes, its ld_token is deliberately cleared to an empty string, so if that same request gets processed again (e.g. a duplicate/stale task-processor delivery), unsigning the now-empty token raises BadSignature: No ":" found in value instead of being a no-op. The PR adds a guard so reprocessing an already-completed request is a safe no-op, and includes a regression test that reproduces the exact crash from the trace without the fix.