fix: remove obsolete Roslyn APIs so warning drift stays visible

[georgepwall1991]

Verdict

Ready: replaces both Roslyn 5.3 obsolete APIs and removes the suppressions that hid future drift.

Evidence

• RED: CI-mode build failed on exactly Workspace.WorkspaceFailed and CodeFixEquivalenceKey after removing CS0618 suppressions.
• GREEN: uses RegisterWorkspaceFailedHandler with scoped disposal and CodeActionEquivalenceKey.
• Existing workspace-failure and Fix All regression tests exercise both paths.
• Full suite: 1,087 passed (775 analyzer/fixer, 303 core, 9 tool).
• CI-mode Release build: 0 warnings, 0 errors.
• Formatter, showcase CFG001-CFG009, package validation, packed-tool smoke test, policy self-tests, and diff check passed.
• Codex review --uncommitted: no actionable findings; it independently reran the full suite and CI build.
• Different-model/Claude review: explicitly waived by user; not represented as passed.

Next action

Merge only after exact-head required checks pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!