LT-22524: Add substring search to Find Lexical Entry

[thejambi]

Quick Summary

Find Lexical Entry now matches typed characters anywhere in a word for queries of 3+ characters, using liblcm's new SearchType.Substring; shorter queries keep the original full-text search. The mode decision lives in a unit-tested SubstringSearchPolicy, and results retain the existing exact -> starts-with -> anywhere ordering.

Depends on sillsdev/liblcm#395

CI-ready checklist

• Commit messages follow .github/commit-guidelines.md (subject ≤ 72 chars, no trailing punctuation; if body present, blank line then ≤ 80-char lines).
• No whitespace warnings locally:

  git fetch origin
  git log --check --pretty=format:"---% h% s" origin/<base>..
  git diff --check --cached

• Builds/tests pass locally (or I've run the CI-style build via Bash script or MSBuild).
• If this is core-developer AI-assisted work, I followed Docs/workflows/ai-pr-workflow.md and ran pr-preflight or the equivalent branch-readiness review before requesting review.
• For any Src/** folders touched, corresponding AGENTS.md files are updated or explicitly confirmed still accurate.

Notes for reviewers (optional)

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This change is