fix: MCP guard, socket host, RPC client, sandbox runner, file launch hardening (#1565)

[ZhuchkaTriplesix]

Refs #1565 (CSV export items are left for a decision: BOM, formula guard and CRLF change the file users get).

• MCP SQL guard: refuse QUERY_TO_XML*, CURSOR_TO_XML, TABLE_TO_XML*, PG_ADVISORY_*, PG_SLEEP*, LO_*, NEXTVAL, SETVAL. EXPLAIN SELECT REPLACE(...) is allowed again; only REPLACE INTO is refused.
• McpSocketHost: at most 16 sockets waiting for their token and 4 KiB before the token line is complete.
• JsonRpcStdioClient.sendRequest: the write is inside the try, so a closed sink does not leave _pending[id] behind.
• SandboxProcessRunner.start: the scratch directory is deleted if detecting bwrap or building the launch command throws.
• FileLaunchService: dropped the duplicate hasPendingTarget; SQLite path comparison is case-insensitive on Windows and macOS.
• Tests for the guard and the socket host.

[ZhuchkaTriplesix]

🔍 Post-Merge Code Review & Verification

Great defense-in-depth security hardening across the MCP and platform subsystems:

1. MCP SQL Guard Refusals (mcp_sql_guard.dart):

   • Thorough coverage against side-effect functions outside transaction semantics (QUERY_TO_XML, PG_ADVISORY_*, PG_SLEEP, LO_*, NEXTVAL, SETVAL).
   • Restricting REPLACE INTO while re-allowing EXPLAIN SELECT REPLACE(...) prevents false-positive query rejections on safe string transformations.

2. DoS Protection on MCP Sockets (mcp_socket_host.dart):

   • Limiting pending unauthenticated connections (maxUnauthenticated: 16) and enforcing a hard cap on handshake line length (maxHandshakeBytes: 4096) mitigates local port-exhaustion attacks.

3. RPC & Process Safety (json_rpc_stdio_client.dart, sandbox_process_runner.dart):

   • Immediate cleanup of _pending[id] upon write failure prevents unbounded completer memory leaks when stdio pipe breaks.
   • Guarantees scratch directory deletion if bubblewrap detection throws.

4. Cross-Platform File Launching (file_launch_service.dart):

   • Case-insensitive comparison _samePath on Windows and macOS aligns with native filesystem semantics for SQLite databases.