fix: use HTTP 404 for invalid session IDs per MCP spec

[ctonneslan]

Summary

• Changes HTTP status code from 400 to 404 when GET/DELETE requests reference an invalid or missing session ID
• Affects simpleStreamableHttp.ts (2 locations), standaloneSseWithGetStreamableHttp.ts (1 location), and everythingServer.ts (2 locations)
• Per the MCP spec, 404 is the correct response when a session ID is not recognized by the server

Fixes #389

Test plan

• Verify conformance tests still pass with updated status codes
• Confirm client transport handles 404 correctly for invalid sessions

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