fix(filesystem): sanitize error messages to prevent path info disclosure

[aptsalt]

Summary

• Sanitize validatePath() error messages in src/filesystem/lib.ts to stop exposing absolute paths and allowed directory lists in error output
• Pin npx examples in src/filesystem/README.md to @0.6.3 to mitigate supply chain risk from unpinned package resolution
• Add 3 tests that explicitly verify error messages don't contain filesystem paths

Context

Addresses the 2 low-severity findings from the AgentAudit security report in #3317:

1. Error messages expose internal path information — validatePath() included the full requested path and the complete allowed directory list in thrown errors. In shared or multi-tenant environments, this could leak directory structure to unauthorized callers.

2. NPX usage without version pinning — Documentation examples used npx -y @modelcontextprotocol/server-filesystem without a version specifier, creating a supply chain risk if a malicious version were published.

Changes

src/filesystem/lib.ts

• Lines 110, 119, 131, 135: Removed interpolated paths from error messages while preserving the descriptive denial reason (e.g. "Access denied - path outside allowed directories")

src/filesystem/README.md

• Pinned both NPX examples to @modelcontextprotocol/server-filesystem@0.6.3

src/filesystem/__tests__/lib.test.ts

• does not leak filesystem paths in access denied errors — verifies error message contains no path or directory info
• does not leak filesystem paths in parent directory errors — verifies parent path is not leaked
• does not leak filesystem paths in symlink denied errors — verifies symlink target path is not leaked

Test plan

• All 48 lib tests pass (npx vitest run __tests__/lib.test.ts)
• Full test suite: 144/145 pass (1 pre-existing Windows-specific failure unrelated to this change)
• tsc --noEmit passes with no type errors
• Verify error messages are still useful for debugging in development

Closes #3317