fix: harden experimental MCP server access

[maxts0gt]

Summary

• restrict experimental MCP scan paths to configured workspace roots, including symlink-aware path resolution
• require an SSE bearer token when listening on non-loopback addresses, with optional OSV_SCANNER_MCP_SSE_TOKEN support
• redact workspace roots from MCP scan output and wire ignore_glob_patterns into scanner exclude patterns

Refs #2785. Thanks @Tobsboy2 for the public report and suggested hardening direction.

Tests

• go test ./cmd/osv-scanner/mcp ./pkg/osvscanner
• UPDATE_SNAPS=true go test ./cmd/osv-scanner/mcp

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[maxts0gt]

@googlebot I signed it!

[Tobsboy2]

Thanks for the fix and the detailed hardening work here, especially the workspace-root restrictions and SSE authentication changes. I appreciate the follow-up and the maintainer review.