fix: 当LTM provider未设置时，回退到全局图片转述模型配置

[Blueteemo]

Summary by Sourcery

Bug Fixes:

• Ensure image captioning uses the default provider when no explicit image_caption_provider_id is set, avoiding failures due to missing provider configuration.

[sourcery-ai]

Hey - I've found 1 issue, and left some high level feedback:

• When reading self.ctx.astrbot_config["provider_settings"], consider using .get("provider_settings", {}) to avoid a KeyError if provider_settings is missing from the configuration.
• The condition if not image_caption_provider_id now mixes None, empty string, and other falsy values; if only unset/empty cases should fall back to the default, consider normalizing or explicitly checking for None/"" to avoid surprising behavior.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- When reading `self.ctx.astrbot_config["provider_settings"]`, consider using `.get("provider_settings", {})` to avoid a `KeyError` if `provider_settings` is missing from the configuration.
- The condition `if not image_caption_provider_id` now mixes `None`, empty string, and other falsy values; if only unset/empty cases should fall back to the default, consider normalizing or explicitly checking for `None`/`""` to avoid surprising behavior.

## Individual Comments

### Comment 1
<location path="astrbot/builtin_stars/astrbot/long_term_memory.py" line_range="72-73" />
<code_context>
         image_caption_provider_id: str,
         image_caption_prompt: str,
     ) -> str:
+        if not image_caption_provider_id:
+            image_caption_provider_id = self.ctx.astrbot_config["provider_settings"].get(
+                "default_image_caption_provider_id"
+            )
</code_context>
<issue_to_address>
**issue:** Guard against missing `provider_settings` key when reading from `astrbot_config`.

Directly indexing `self.ctx.astrbot_config["provider_settings"]` will raise a `KeyError` if that section is absent in partially initialized or customized configs. Use a safe default, e.g. `self.ctx.astrbot_config.get("provider_settings", {})`, before calling `.get("default_image_caption_provider_id")`.
</issue_to_address>

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[gemini-code-assist]

Code Review

This pull request introduces a fallback mechanism to retrieve a default image caption provider ID from the configuration when it is not explicitly provided. However, the implementation contains a critical AttributeError as it attempts to access undefined attributes self.ctx and self.astrbot_config instead of using self.context.get_config(). Furthermore, the new logic is currently unreachable due to existing constraints in the cfg() method, and feedback suggests refactoring the code for better reuse and adding unit tests to verify the behavior.