Headers Cookie not combined with Managed Cookie cookies

### Expected Behavior

The expected behavior is that the cookie header in the requestis added with stored cookies resulting in a single cookie, semi-colon deliniated, in the request. For example this is how cURL does it:

```terminal
curl --request POST \
--url https://something.com/api \
--header 'Content-Type: application/json' \
--header 'Cookie: session_id=' \
--cookie 'AWSALB=<32bit string>; AWSALBCORS=<32bitstring>' \
--data '{}'
```
The above cURL was generated by Insomnia but succeeds in cURL.

### Actual Behavior

Insomnia creates multiple cookies in the request and only the first one is read by the server. See Insomnia console output of the same query:

```terminal
> POST /api HTTP/2
> Host: https://something.com
> cookie: AWSALBCORS=<32bitstring>; AWSALB=<32bit string>
> content-type: application/json
> cookie: session_id=
> accept: */*
> content-length: 2
```

### Reproduction Steps

Make an API call with a cookie in the headers section of the request and a stored cookie in the managed cookies. The console output shows multiple cookies being created. Export the same query for cURL and it will concat all the cookies into one cookie.

### Is there an existing issue for this?

- [x] I have searched the [issue tracker](https://www.github.com/Kong/insomnia/issues) for this problem.

### Which sync method do you use?

- [ ] Git sync.
- [ ] Insomnia Cloud sync.
- [x] Local only

### Additional Information

_No response_

### Insomnia Version

v12.3.0

### What operating system are you using?

macOS

### Operating System Version

macOS Tahoe Version 26.2

### Installation method

homebrew

### Last Known Working Insomnia version

_No response_

Reproduce the request in Insomnia with one manually supplied header cookie and stored managed cookies, then compare its console output with the exported cURL request. Trace the request construction entry point responsible for cookies; done means all cookies are sent in one semicolon-delimited Cookie header so the server reads them together.

Written by the indexing model from the issue text.