feat(memory): add optional Milvus vector backend

[dajiaohuang]

What does this PR do?

Adds the optional Milvus follow-up requested in #2956 after the VectorBackend abstraction landed in #3021.

• Adds a lazy-imported MilvusVectorBackend for Milvus Lite, Milvus Server, and Zilliz Cloud, with an explicit schema, COSINE/AUTOINDEX search, workspace isolation, metadata filters, collection validation, and embedding-dimension checks.
• Keeps SQLite as the zero-dependency authoritative store and keeps FTS5 unchanged. A versioned durable outbox backfills existing SQLite vectors, retries idempotent writes/deletes, avoids losing concurrent updates, and falls back to SQLite until Milvus is healthy and fully synchronized.
• Adds milvus and milvus-lite optional extras plus runtime configuration, token/URI log redaction, rebuild preflight/repopulation, backend health status, and English/Chinese/Japanese documentation.
• Preserves CowAgent's core requires-python >=3.7; the optional server extra requires Python 3.9+, while current Milvus Lite requires Python 3.10+.

Type of change

• Bug fix
• New feature
• Docs
• Refactor / chore

Testing

• pip install -e .[milvus-lite] in an isolated environment, followed by the focused backend, real Milvus Lite, and rebuild suites: 17 passed.
• Focused unit/config/security suite: 35 passed.
• Memory/knowledge/search regression matrix: 139 passed, 1 skipped, 14 failed. The same command on a clean origin/master worktree produced 127 passed, 14 failed; all 14 failures are pre-existing Windows/environment issues (symlink privileges, path separators, ripgrep CJK output, missing web.py) or existing workspace/recovery failures.
• python -m compileall -q agent bridge tests
• Ruff on the changed memory/backend/test modules
• git diff --check origin/master...HEAD

Checklist

• I have read the Contributing Guide
• I tested this change locally
• Code comments and docs are in English
• Linked related issue (if any): closes Proposal: pluggable memory vector backend and optional Milvus support #2956

Closes #2956