#19502: Docker: install extension workspace deps for memory-lancedb

## Summary - fix Docker install flow so bundled extension dependencies are installed - copy `extensions/` before install and run workspace-aware install ## Changes - `Dockerfile` - `COPY extensions ./extensions` before dependency install - `RUN pnpm -r install --frozen-lockfile` ## Why this is minimal - avoids broad Docker/build refactors - targets only the packaging gap that caused `memory-lancedb` runtime dependency misses ## Validation - built Docker image locally - confirmed plugin-context resolution for `openai` and `@lancedb/lancedb` - loaded `extensions/memory-lancedb/index.ts` in container - triggered `memory_store` with invalid key and got upstream `401 Incorrect API key provided`, confirming request path is functional Fixes #19466 <!-- greptile_comment --> <h3>Greptile Summary</h3> This PR fixes Docker builds by ensuring extension dependencies are installed correctly. The change copies the `extensions/` directory before running `pnpm install` and uses the `-r` (recursive) flag to install all workspace package dependencies, including those from bundled extensions like `memory-lancedb`. This is a cleaner approach than the previously-reverted fix that attempted to install extension dependencies after the build as a separate step. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - The changes are minimal and well-targeted: copying extensions before install and using pnpm's recursive flag is the correct approach for workspace package management. This fixes a real issue (missing runtime dependencies) in a clean, maintainable way. The author validated the fix locally and confirmed functional behavior. The approach is superior to the previously-reverted band-aid fix that installed dependencies after the build. - No files require special attention _{Last reviewed commit: b657092} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->