Reject invalid LSP coordinates as InvalidParams

[Widthdom]

Summary

• Centralize LSP Position and Range structural validation before document, live-text, and index access.
• Return JSON-RPC -32602 for malformed, negative, overflowing, missing, or reversed request coordinates while preserving valid UTF-16 and EOF behavior.
• Reject invalid ranged didChange notifications before they can mutate accepted live text.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false — passed with 0 warnings and 0 errors.
• Issue Reject negative LSP positions as InvalidParams #4869 regression tests on net8.0 and net9.0 — 3 passed on each target.
• Full net8.0 suite — 10,777 passed, 7 expected skips, 0 failed.
• Full net9.0 suite — 10,302 passed, 420 expected skips; one unrelated watcher timeout is tracked in Investigate net9 ancestor .gitignore watcher reconciliation timeout #4966.
• dotnet format CodeIndex.sln --verify-no-changes --no-restore — passed.
• dotnet run --project tools/CodeIndex.Changelog -- check — validated all 41 fragments.
• Commit-aware index refresh and status --check --json — fresh, clean, and matched to HEAD.
• Codex adversarial review of origin/main..HEAD — No blocking/actionable issues found.

Documentation and changelog

• Updated the English and Japanese LSP contracts in DEVELOPER_GUIDE.md.
• Updated the English and Japanese regression-test guidance in TESTING_GUIDE.md.
• Added changelog.d/unreleased/4869.fixed.md.

Follow-up candidates

• Investigate net9 ancestor .gitignore watcher reconciliation timeout #4966

Fixes #4869