Assistant: add setting for user-defined model listing

[sharon-wang]

Summary

• addresses Configure static list of available models (names, metadata, token limits, etc) #9971
• add positron.assistant.configuredModels setting to allow users to statically configure model settings per provider, which overrides dynamic model resolution
• update and add tests around model resolution

Approach

• each provider's resolveModels() method now calls retrieveModelsFromConfig() to check for statically defined models via positron.assistant.configuredModels or hardcoded models, then retrieveModelsFromApi() to dynamically retrieve the model listing if applicable for the provider
• move some duplicated code to shared util files modelDefinitions.ts and modelResolutionHelpers.ts
• continues some refactoring to incrementally work towards Assistant: reorg/refactor provider handling #10532

Release Notes

New Features

• Assistant: add initial support for user-defined model listing (Configure static list of available models (names, metadata, token limits, etc) #9971)

QA Notes

• Added new and updated existing Positron Assistant extension integration tests (run locally with npm run test-extension -- -l positron-assistant)
• The setting should work for all supported providers. Note that the currently selected model sometimes may not change when the setting is updated -- this is a known issue Assistant: The model provider and model selections can get into a bad state #9443.

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical

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[sharon-wang]

I've removed this as we no longer use the hardcoded model listing, as we've switched to using the Bedrock SDK to dynamically retrieve the models.