chore: Added the new gemini exp-1121 and learnlm-1.5 models

[CXwudi]

Summary

Gemini added more experimental models: ai.google.dev/gemini-api/docs/models/experimental-models

Also, I aligned the context size to 32767 as shown in the Gemini Studio https://aistudio.google.com/

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Close issue syntax: Fixes #<issue number> or Resolves #<issue number>, see documentation for more details.

Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• (no need) I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• (no need) I've updated the documentation accordingly.
• (no need, no code changes) I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods