OllamaSharp registration fixes

[aaronpowell]

Refactor OllamaSharp client so that the MEAI types will resolve the correct OllamaSharp API and thus the right default model.

Fixes #310

Closes #<ISSUE_NUMBER>

PR Checklist

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• PR doesn't include merge commits (always rebase on top of our main, if needed)
• New integration
  • Docs are written
  • Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that)
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Contains NO breaking changes
• Every new API (including internal ones) has full XML docs
• Code follows all style conventions

Other information

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
CommunityToolkit.Aspire.EventStore	100%	100%	46	✔
CommunityToolkit.Aspire.Hosting.ActiveMQ	60%	24%	104	❌
CommunityToolkit.Aspire.Hosting.ActiveMQ.MassTransit	1%	0%	14	❌
CommunityToolkit.Aspire.Hosting.Azure.DataApiBuilder	100%	100%	22	✔
CommunityToolkit.Aspire.Hosting.Azure.StaticWebApps	100%	100%	28	✔
CommunityToolkit.Aspire.Hosting.Bun	81%	71%	54	✔
CommunityToolkit.Aspire.Hosting.Deno	84%	75%	72	✔
CommunityToolkit.Aspire.Hosting.EventStore	90%	71%	62	✔
CommunityToolkit.Aspire.Hosting.Golang	94%	50%	16	✔
CommunityToolkit.Aspire.Hosting.Java	98%	71%	58	✔
CommunityToolkit.Aspire.Hosting.Meilisearch	61%	27%	94	➖
CommunityToolkit.Aspire.Hosting.NodeJS.Extensions	90%	68%	92	✔
CommunityToolkit.Aspire.Hosting.Ollama	65%	64%	198	➖
CommunityToolkit.Aspire.Hosting.Python.Extensions	67%	46%	56	➖
CommunityToolkit.Aspire.Hosting.Rust	94%	83%	16	✔
CommunityToolkit.Aspire.Hosting.SqlDatabaseProjects	72%	64%	58	➖
CommunityToolkit.Aspire.MassTransit.RabbitMQ	100%	100%	30	✔
CommunityToolkit.Aspire.Meilisearch	97%	92%	68	✔
CommunityToolkit.Aspire.OllamaSharp	87%	82%	68	✔
Summary	76% (2319 / 3046)	63% (577 / 912)	1156	➖

Minimum allowed line rate is 60%

Copilot reviewed 3 out of 3 changed files in this pull request and generated no suggestions.

Comments skipped due to low confidence (1)

tests/CommunityToolkit.Aspire.OllamaSharp.Tests/ConformanceTests.cs:33

• The use of 'string?' in 'KeyValuePair<string, string?>' is unnecessary if the values are always provided and never null. Consider changing it to 'string'.

new KeyValuePair<string, string?>("ConnectionStrings:{key ?? 'ollama'}", $"Endpoint={endpoint}"),