fix(blocks): validate non-empty input in AIConversationBlock before LLM call (#12545)

### Why / What / How

**Why:** When `AIConversationBlock` receives an empty messages list and
an empty prompt, the block blindly forwards the empty array to the
downstream LLM API, which returns a cryptic `400 Bad Request` error:
`"Invalid 'messages': empty array. Expected an array with minimum length
1."` This is confusing for users who don't understand why their agent
failed.

**What:** Add early input validation in `AIConversationBlock.run()` that
raises a clear `ValueError` when both `messages` and `prompt` are empty.
Also add three unit tests covering the validation logic.

**How:** A simple guard clause at the top of the `run` method checks `if
not input_data.messages and not input_data.prompt` before the LLM call
is made. If both are empty, a descriptive `ValueError` is raised. If
either one has content, the block proceeds normally.

### Changes

- `autogpt_platform/backend/backend/blocks/llm.py`: Add validation guard
in `AIConversationBlock.run()` to reject empty messages + empty prompt
before calling the LLM
- `autogpt_platform/backend/backend/blocks/test/test_llm.py`: Add
`TestAIConversationBlockValidation` with three tests:
- `test_empty_messages_and_empty_prompt_raises_error` — validates the
guard clause
- `test_empty_messages_with_prompt_succeeds` — ensures prompt-only usage
still works
- `test_nonempty_messages_with_empty_prompt_succeeds` — ensures
messages-only usage still works

### Checklist

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  - [x] Lint passes (`ruff check`)
  - [x] Formatting passes (`ruff format`)
- [x] New unit tests validate the empty-input guard and the happy paths

Closes #11875

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Co-authored-by: Zamil Majdy <zamil.majdy@agpt.co>