fix: preserve self-hosted model endpoints on model changes

[geilwerk]

Summary

• preserve self-hosted LLM endpoints when switching models
• preserve self-hosted embedding endpoints during model changes
• add regression coverage for Ollama/self-hosted and cloud-only updates

Context

Self-hosted Letta deployments can store custom model and embedding endpoints such as Ollama. Updating the model with a minimal payload was causing those saved endpoints to be dropped, which broke agents using Docker plus Ollama setups.

Testing

• npm run test:run -- src/tools/letta-api.test.ts

[Copilot]

Pull request overview

Fixes a regression where updating an agent’s model with a minimal payload on self-hosted Letta servers could drop persisted custom LLM/embedding endpoints (e.g., Ollama), breaking agents configured with custom endpoints.

Changes:

• Adds self-hosted-aware preservation logic to keep llm_config.model_endpoint and embedding_config.embedding_endpoint intact when switching models.
• Refactors base URL handling to be read dynamically from process.env.LETTA_BASE_URL (instead of a module-level constant).
• Adds Vitest regression coverage for self-hosted endpoint preservation and cloud minimal-update behavior.

Reviewed changes

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

File	Description
src/tools/letta-api.ts	Preserves self-hosted LLM/embedding endpoint configs during model updates; introduces helper functions for handle parsing and preservation decisions.
src/tools/letta-api.test.ts	Adds regression tests covering self-hosted Ollama preservation and cloud minimal update payloads.

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[geilwerk]

Addressing the review feedback is pushed.

Changes in the follow-up:

• restored the minimal cloud update path to avoid the extra retrieve round-trip
• added coverage for the self-hosted provider-mismatch case

Validation:

• npm run test:run -- src/tools/letta-api.test.ts
• npm run build