Add Google temporary chat mode with metadata fallback

[Vigno04]

Summary

• add configurable gemini.chat_mode with normal and temporary options
• wire Google temporary chat mode (temporary=true) into send paths
• skip metadata reuse when temporary mode is enabled
• add fallback to internal history replay when metadata-backed Google chat cannot be found
• document new config options in README and config comments

Notes

• temporary mode now keeps account history clean on Google side
• normal mode retains existing reuse behavior with safer fallback

[Copilot]

Pull request overview

Adds configurable support for Google “temporary chat” mode and improves resilience when metadata-based chat reuse fails.

Changes:

• Introduces gemini.chat_mode (normal vs temporary) and gemini.fallback_to_internal_on_missing_chat configuration options.
• Skips LMDB metadata session reuse when chat_mode=temporary, and plumbs a temporary flag into Gemini send paths.
• Adds a retry path that falls back to internal history replay when metadata-backed chat reuse cannot be restored.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
config/config.yaml	Adds new Gemini configuration keys for chat mode and fallback behavior.
app/utils/config.py	Extends GeminiConfig schema with chat_mode and fallback toggle.
app/server/chat.py	Implements temporary-mode behavior, safer session reuse handling, and internal fallback retry logic.
README.md	Documents new configuration options and environment variables.

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[Nativu5]

Hi @Vigno04, thanks for your contribution. Providing a way to utilize temporary session is really helpful. I'm looking forward to merging it once we resolved all the comments.

[luuquangvu]

Temporary chat mode doesn't simply work by disabling all chat metadata for all chats and resending the entire content. If you did that with clients that have large prompts like Home Assistant, you'd likely encounter no response from Gemini after only the second or third request.

[Vigno04]

i've added compaction and file context as options

[Nativu5]

Generally good with serval remaining comments to be resolved. Thanks!