fix: add checks to return None if STT or TTS providers are disabled in config

[machinad]

当用户在模型服务商添加了tts或stt服务，但是因为没有安装依赖等导致服务不可用时，这时用户如果在普通配置-ai配置-启动文本转语音(打开)-默认文本转语音模型(选择例如edge-tts)后会发现服务无法运行，此时如果用户只是单纯关掉tts\stt的总开关而没有清除默认模型选择项的话，就会导致astrbot/core/provider/manager.py的provider_id(例如edge-tts)仍然会保留，这时消息平台每一次发起对话请求后台都会输出日志:"没有找到 ID 为edge-tts的提供商，这可能是由于您修改了提供商（模型）ID 导致的。"即便用户已经关掉了tts/stt的总开关或者已经在模型服务商处已经删除掉了模型服务，但因为在ai配置-启动文本转语音(关闭)-默认文本转语音模型处未清除掉模型名称而导致日志仍在在输出:"没有找到 ID 为edge-tts的提供商，这可能是由于您修改了提供商（模型）ID 导致的。"

Modifications / 改动点

在astrbot/core/provider/manager.py的get_using_provider函数中分别对tts和stt提供商引入tts\stt的总开关判断，当总开关关闭时就直接不获取tts/stt的提供商实例了，毕竟总开关都关闭了嘛。

• This is NOT a breaking change. / 这不是一个破坏性变更。

Screenshots or Test Results / 运行截图或测试结果

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Checklist / 检查清单

• 😊 If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
  / 如果 PR 中有新加入的功能，已经通过 Issue / 邮件等方式和作者讨论过。

• 👀 My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
  / 我的更改经过了良好的测试，并已在上方提供了“验证步骤”和“运行截图”。

• 🤓 I have ensured that no new dependencies are introduced, OR if new dependencies are introduced, they have been added to the appropriate locations in requirements.txt and pyproject.toml.
  / 我确保没有引入新依赖库，或者引入了新依赖库的同时将其添加到 requirements.txt 和 pyproject.toml 文件相应位置。

• 😮 My changes do not introduce malicious code.
  / 我的更改没有引入恶意代码。

Summary by Sourcery

Bug Fixes:

• Prevent repeated provider-not-found errors when STT or TTS global switches are off but a default provider ID remains configured.

[sourcery-ai]

Hey - I've left some high level feedback:

• The new enable checks treat a missing enable key the same as False, which may change behavior for existing configs that omit this flag; consider using config[...].get('enable', True) or checking explicitly for False to preserve current defaults.
• The STT and TTS branches now share very similar enable/provider_id logic; consider extracting a small helper to reduce duplication and keep get_using_provider easier to maintain.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The new `enable` checks treat a missing `enable` key the same as `False`, which may change behavior for existing configs that omit this flag; consider using `config[...].get('enable', True)` or checking explicitly for `False` to preserve current defaults.
- The STT and TTS branches now share very similar enable/`provider_id` logic; consider extracting a small helper to reduce duplication and keep `get_using_provider` easier to maintain.

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[gemini-code-assist]

Code Review

This pull request updates the provider manager to check for an 'enable' flag within the speech-to-text and text-to-speech configuration settings before attempting to retrieve a provider. If the feature is disabled, the method now correctly returns None. I have no further feedback to provide.