Add Accept header for HTTP MCP backend session establishment

[Copilot]

The safeinputs HTTP backend returns "Session not found" (404) because the gateway doesn't send the required Accept header during session establishment.

Changes

• Added Accept: application/json, text/event-stream header to HTTP MCP requests
  • initializeHTTPSession(): Ensures proper content negotiation during session creation
  • sendHTTPRequest(): Maintains consistency for all HTTP backend communication

Context

HTTP MCP backends like safeinputs require explicit content type negotiation to establish sessions. Without the Accept header, the backend cannot determine the response format, leading to session initialization failures.

// Before: Missing Accept header
httpReq.Header.Set("Content-Type", "application/json")
httpReq.Header.Set("Mcp-Session-Id", tempSessionID)

// After: Content negotiation enabled
httpReq.Header.Set("Content-Type", "application/json")
httpReq.Header.Set("Accept", "application/json, text/event-stream")
httpReq.Header.Set("Mcp-Session-Id", tempSessionID)

This aligns with the safeinputs server's documented initialization flow, which captures the session ID from response headers for subsequent authenticated requests.

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