fix: escape qdrant-find entry fields

[mikemikimike]

Summary

Fixes #179.

Stored content and metadata are interpolated directly into the XML-like response returned by qdrant-find. If either value contains structural tags such as </content><metadata>, the response becomes ambiguous and a consumer cannot reliably distinguish stored values from the server-generated boundaries.

This change:

• escapes &, <, and > in content and serialized metadata before interpolation;
• preserves the existing <entry>, <content>, and <metadata> response structure;
• adds parameterized offline regression coverage for plain text, &/</>, metadata=None, nested JSON metadata, and injected structural tags.

The formatter is a pure transformation: it does not mutate entries, shared state, or connector state, so the fix introduces no new race or partial-update path.

Verification

• uv run --frozen pytest tests/test_mcp_server.py -q — 3 passed
• uv run --frozen ruff format --check . — passed
• uv run --frozen ruff check . — passed
• git diff --check — passed
• uv run --frozen pytest -q — interrupted after more than 70 seconds without output while entering the integration path; no failure was reported before interruption
• uv run --frozen mypy src tests — existing repository errors (9 errors in fastembed.py, qdrant.py, common/filters.py, and existing mcp_server.py filter typing)

The focused tests exercise the exact formatter used by qdrant-find; a full Qdrant/MCP integration test was not added because the tool closure is local to setup_tools() and introducing test-only production seams would broaden this focused fix.