fix(agentic): validate RecallFile pagination inputs

[wutongyuonce]

Fixes #662

Summary

Validate RecallFile pagination inputs at the shared AgenticMixin service boundary before repository access.

Root cause

The pagination feature added _decode_cursor() and forwarded limit directly to list_recall_files_page(). Malformed cursors leaked decoder and tuple-unpacking exceptions, while limit=0 reached backend implementations without a shared contract.

Changes

• Reject limit < 1 with ValueError("limit must be greater than zero").
• Normalize malformed Base64, UTF-8, JSON, shape, and field-type failures to ValueError("invalid cursor").
• Add regression coverage for InMemory and SQLite backends.
• Keep the change in the shared service entry point; no repository-specific changes or new dependencies.

Validation

• tests/test_agentic.py -q: 32 passed
• Ruff check: passed
• mypy for src/memu/app/agentic.py: passed
• git diff --check: passed

This PR is based on the latest upstream main at commit 37d2efa.

[MrXnneHang]

LGTMeow 🐾