Customize knowledge creation behavior in Playground via theme db

[ckelly17]

Description

This PR contains allows for two customizations of UI and back-end behavior related to knowledge (vectors) in Playground:

1. Allows disabling of automatic registration of vectors as MCPs upon creation through Playground. By default, we run MakeEngineMCP while creating vectors. This PR creates the enableKnowledgeMCP boolean flag set in theme to override this if desired. This is may be important in some implementations, since the engine MCP would be attached to any chat using the vector, creating potential collisions with other MCPs designed for handling vector databases, reading documents, etc.
2. Creates an option to disable the user selection of an embedding model for vector creation. Since this may be too technical for many users, it may be desired in some implementations. By setting allowEmbeddingOptions to false in theme, this allows the option to hide that dropdown on the UI. If this is hidden, the vector database will be created using the new defaultEmbedderId value from theme or select the first model available as returned by MyEngines.

Changes Made

• Added new values to theme store
• Implemented conditional logic based on new theme values in packages/playground/src/components/knowledge/new-knowledge-mcp-overlay.tsx

How to Test

If you wish to test disabling MCP creation and embedding model selection, set enableKnowledgeMCP and allowEmbeddingOptions to false in theme. To test, create a new knowledge store in playground. You should see:

1. No embedding model selection
2. No MakeEngineMCP pixel in network calls
3. No MCP tag applied to the vector
4. When using the vector in a chat, you should not see the default vector MCP invoked

To regression test current behavior, set these as true in theme or do not add them to theme. You should see that vectors become MCPs by default and users may select embedders of their choosing.

Screenshots

Notes

theme.local added in vite config can be ignored. Used for local testing

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