Add Apple Foundation Models support (Apple FM)

[jimscard]

Resolves #2154

Add Apple Foundation Models support (Apple FM)

Summary

This PR adds support for Apple Foundation Models (OpenAI-compatible) as a provider in Fabric. It includes robust handling for servers that return SSE (text/event-stream) for non-stream responses by parsing SSE "data:" chunks and concatenating content. A secondary fallback resends a single concatenated user message when SSE parsing yields empty output.

Key changes

• Register Apple Foundation Models provider (default base URL: http://localhost:1976/v1). API key optional for local servers.
• Implement direct HTTP fallback for Chat Completions when the SDK returns no choices or errors.
• parseSSEAndConcat: robust SSE parser using bufio.Reader to avoid scanner limits and extract both JSON delta/message content and raw text data.
• Secondary fallback: resend single concatenated user message when SSE parse yields empty output for some server behaviors.
• Unit tests for SSE parsing.
• README updated with server guidance (fm serve example), tmux snippet, and host-binding warning.

Lint & cleanup (follow-up)

• Replaced inefficient string concatenation in a loop with strings.Builder for the concatenated-message fallback.
• Simplified HasPrefix+TrimPrefix to strings.CutPrefix in SSE parsing.
• These cleanups address CI/linter warnings and were committed in 4ce138c (fix(openai): optimize SSE parsing).

Files touched (high level)

• internal/plugins/ai/openai_compatible/providers_config.go
• internal/plugins/ai/openai/chat_completions.go
• internal/plugins/ai/openai/chat_completions_test.go
• README.md

Testing performed locally

• Ran full test suite: go test ./... (all packages report OK locally)
• Built and ran a sample YouTube summary against a local Apple FM server and validated output.
• Verified non-stream completions against an Apple FM server returning text/event-stream; SSE parsing and concatenated-message fallback yield plain text.

How to test

1. Start Apple FM server (requires macOS Golden Gate 27.0 beta 1+):

   # Start in background via tmux (recommended)
   tmux has-session -t fm_server || tmux new -d -s fm_server fm serve --host 0.0.0.0 --port 1976

2. Verify vendor and models:

   ./fabric --listvendors
   ./fabric --listmodels --vendor "Apple Foundation Models"

3. Run a non-stream sample to exercise SSE fallback:

   ./fabric -y https://youtu.be/piKWmRBMgRU -p "youtube_summary" --vendor "Apple Foundation Models" --model pcc

Notes for reviewers

• Please review parseSSEAndConcat for correctness and edge cases.
• Consider Git LFS for future large artifacts if the project wishes to store binaries in-repo.

Recent commits on add-fm (most recent first):

$(git log add-fm -n 12 --pretty=format:"- %h %s (%an)")