fix(examples): use 404 status for invalid session IDs

[corvid-agent]

Summary

• Changes HTTP status code from 400 to 404 for invalid/missing session ID responses across all streamable HTTP examples
• Per the MCP spec: "When a client receives HTTP 404 in response to a request containing an Mcp-Session-Id, it MUST start a new session"
• Updated error messages from "Bad Request" to "Not Found" for consistency

Files changed

• jsonResponseStreamableHttp.ts — POST handler
• simpleStreamableHttp.ts — POST, GET, DELETE handlers
• standaloneSseWithGetStreamableHttp.ts — POST, GET handlers
• simpleTaskInteractive.ts — POST, GET, DELETE handlers
• elicitationFormExample.ts — POST, GET, DELETE handlers
• elicitationUrlExample.ts — POST, GET, DELETE handlers

Closes #389

Test plan

• Verify all session-related 400 responses changed to 404
• Non-session 400 responses (e.g., missing parameters) left unchanged

🤖 Generated with Claude Code

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