This adds support for running self-hosted Sweep Next Edit edit prediction models through Zed's OpenAI-compatible provider. The target use case is a local deployment based on sweepai/sweep-next-edit-1.5B, using the rewrite-window prompting approach described in OSS Next Edit. Personal user testing shows impressive quality from this small edit prediction model.

This is valuable because it allows more flexibility for Zed users to self-host state of the art next edit prediction models!

This PR addresses the self-hosted workflow discussed in #50929.
Note that the self-hosted model path needed more than just a new dropdown value. Zed has to build the rewrite-window prompt shape the model expects, map the rewritten window back into anchored edits, and make prompt_format: "infer" work with the filename-style model identifiers that llama-server local server reports.

The main changes are:

• add a Sweep prompt format for OpenAI-compatible edit prediction providers and route it to a dedicated SweepPrompt model path
• build Sweep rewrite-window prompts from the active cursor window, recent change history, and related file excerpts
• convert rewrite responses back into anchored edits and suppress no-op rewrites
• add fixed cursor-window extraction used by the Sweep rewrite prompt path
• recognize sweep-next-edit model names during infer, including filename-style identifiers such as sweepai_sweep-next-edit-1.5B_sweep-next-edit-1.5b.q8_0.v2.gguf
• reject reserved Sweep prompt tokens before sending malformed requests to a self-hosted server
• update the docs to describe the actual Sweep rewrite-window prompt format

Local setup used for manual verification:

llama-server \
  --hf-repo sweepai/sweep-next-edit-1.5B \
  --hf-file sweep-next-edit-1.5b.q8_0.v2.gguf \
  --ctx-size 8192 \
  --host 127.0.0.1 \
  --port 8080

{
  "edit_predictions": {
    "provider": "open_ai_compatible_api",
    "open_ai_compatible_api": {
      "api_url": "http://127.0.0.1:8080/v1/completions",
      "model": "sweepai_sweep-next-edit-1.5B_sweep-next-edit-1.5b.q8_0.v2.gguf",
      "prompt_format": "infer",
      "max_output_tokens": 512
    }
  }
}

The request that produced this PR included screenshots of the OpenAI-compatible provider configuration and the prompt format dropdown with Sweep selected.

Tests added in this branch:

• test_sweep_prompt_format_routes_to_sweep_prompt_model
• test_fixed_line_window_around_cursor_start_middle_and_end
• test_sweep_prompt_request_prediction_diffs_rewritten_window_into_anchored_edits
• test_sweep_prompt_request_prediction_returns_none_for_identical_rewrite
• test_original_window_for_current_window_uses_latest_pre_edit_snapshot
• test_original_window_for_current_window_returns_none_without_matching_history
• test_recent_change_block_from_event_formats_original_and_updated_sections

Verification run locally:

• cargo test -p zed sweep_prompt_format_routes
• cargo test -p zed subscribe_uses_stale_provider_config
• cargo test -p edit_prediction sweep_prompt
• cargo test -p edit_prediction fixed_line_window_around_cursor_start_middle_and_end
• cargo test -p edit_prediction test_sweep_prompt_request_prediction_diffs_rewritten_window_into_anchored_edits
• cargo test -p edit_prediction test_sweep_prompt_request_prediction_returns_none_for_identical_rewrite
• ./script/clippy -p zed -p edit_prediction -p settings_content

Release Notes:

• Added support for self-hosted Sweep Next Edit models in OpenAI-compatible edit predictions, including the sweep prompt format and infer detection for sweep-next-edit model names.