refactor: remove legacy Snyk Code client

[fsargent]

Summary

• Remove the unused TypeScript Snyk Code implementation and @snyk/code-client dependency.
• Keep Snyk Code on the native code-client-go workflow.
• Reject accidental Snyk Code use through legacy TypeScript entry points.
• Remove tests for the retired implementation and keep native workflow coverage.

This replaces the stale PR #6944. File Upload API support and user-journey coverage have since merged in PR #7061 and PR #7064.

Test plan

• npm run lint
• npx jest --runInBand test/jest/unit/ecosystems.spec.ts test/jest/unit/validate-fix-command-is-supported.spec.ts
• npm run build:dev
• Snyk Code scan. No finding affects changed code.

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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

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