chore: remove deprecated AI Explain surface from llm package

[pasiunaite]

Description

Follow-up cleanup to #150. Agent Fix populates explanation on every fix it returns and the client passes it through on AutofixUnifiedDiffSuggestion.Explanation, so the AI Explain client in the llm package has no remaining purpose.

Removed:

• ExplainWithOptions, Explain, ExplainResult, AIRequest
• runExplain, explainRequestBody, prepareDiffs, getOrderedResponse, defaultEndpointURL
• ExplainOptions, Explanations, explainResponse, explainVulnerabilityRequest, explainFixRequest, explanationLength and its SHORT/MEDIUM/LONG constants
• the SnykLLMBindings interface and its PublishIssues, which panicked with implement me and only existed to model the LLM/explain era
• binding_smoke_test.go, which only exercised Explain

DeepCodeLLMBinding is now just GetAutofixDiffs + SubmitAutofixFeedback.

OutputFormat, its constants and WithOutputFormat are deliberately kept. They no longer affect behaviour, but snyk-ls and remy-cli-extension still pass llm.WithOutputFormat(llm.HTML), and removing them would force a simultaneous change in both. They can go in a later pass once consumers drop the option.

Blocked on

Do not merge until snyk-ls has removed its AI Explain path (IDE-2487). snyk-ls main is the only remaining consumer, at infrastructure/code/ai_fix_handler.go (EnrichWithExplain calling ExplainWithOptions). An org-wide code search for ExplainWithOptions and llm.ExplainOptions returns that single call site and nothing else.

This is a breaking change to a v1 module, so it also needs a call on how to release it — every consumer is first-party, so a minor bump may be acceptable, but that is worth an explicit decision rather than an assumption.

Checklist

• Tests added and all succeed
• Linted
• README.md updated, if user-facing — no mentions of explain in README or docs

gofmt, golangci-lint (0 issues), go build ./..., go vet ./llm/... and go test ./... all pass. go mod tidy produces no drift. No generated mocks reference the removed interfaces.

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[snyk-io]

✅ Snyk checks have passed. No issues have been found so far.

Status	Scan Engine	Critical	High	Medium	Low	Total (0)
✅	Open Source Security	0	0	0	0	0 issues
✅	Licenses	0	0	0	0	0 issues
✅	Code Security	0	0	0	0	0 issues

💻 Catch issues earlier using the plugins for VS Code, JetBrains IDEs, Visual Studio, and Eclipse.