fix: handle inherited property names in MCP server listings

[lokesh0186]

Fixes #607

Use null-prototype dictionaries for the two server-grouping sites in ToolSearch. Names such as constructor, toString, and hasOwnProperty otherwise resolve to inherited properties, so adding a group member throws before tool search can initialize or a server preview can be returned.

The patch leaves the search algorithm, relevance scores, tool loading, and invocation behavior unchanged. Tests use an in-memory registry and the real local search path, with no network requests.

Validation:

• Unpatched main: nine of eleven new regressions fail; both ordinary controls pass.
• Patched main: all eleven regressions and all 125 existing ToolSearch tests pass.
• npx tsc --noEmit: passes.
• Targeted ESLint: zero errors; the pre-existing strict-boolean warning at ToolSearch.ts:1352 remains unchanged.
• Published npm 4.0.4 and 4.0.1 reproduce the grouping errors, using current-lockfile dependencies for the bundle checks.

The full SDK suite, model/API integration checks, and a full LibreChat deployment have not been run locally.

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