Add structured filesystem startup validation errors

[davidahmann]

Problem
Filesystem server startup failures for missing or invalid roots were only emitted as free-form stderr text, which makes automated classification and audit receipts brittle.

Why now
Issue #3512 asked for a machine-readable startup taxonomy. Operators need stable error codes when validating fail-closed root configuration in CI and wrapper processes.

What changed

• emit structured JSON-line startup validation events alongside existing human-readable stderr
• cover argv validation failures for inaccessible paths, non-directories, and "no accessible directories"
• reuse the structured emission for MCP roots validation paths

Validation

• npm run --workspace @modelcontextprotocol/server-filesystem build
• npm run --workspace @modelcontextprotocol/server-filesystem test -- __tests__/startup-validation.test.ts

Refs #3512

[davidahmann]

User impact: Wrapper processes can now classify filesystem startup failures deterministically instead of scraping only free-form stderr.
Change: The filesystem server now emits structured JSON-line startup validation events alongside the existing human-readable messages and covers the argv validation cases in tests.
Validation:

• npm run --workspace @modelcontextprotocol/server-filesystem build passed
• npm run --workspace @modelcontextprotocol/server-filesystem test -- __tests__/startup-validation.test.ts passed

Inspired by research context: CAISI publishes independent, reproducible AI agent governance research: https://caisi.dev