fix: preserve equals in curl form imports

[immanuwell]

Related: #9412, #9434, #9198

This is a small follow-up for cURL import.
-d style payloads were fixed already, but -F/--form was still kinda busted. The form parser used split('='), so multipart values and file paths got chopped after the first extra =.

Repro

1. Create -> Import From cURL
2. Paste curl -X POST https://example.com -F 'token=abc=def=='
3. Or paste curl -X POST https://example.com -F 'file=@/tmp/a=b.txt'
4. Import

Before

• token becomes abc
• file path becomes /tmp/a

After

• token stays abc=def==
• file path stays /tmp/a=b.txt

This is real-world enough, not some unicorn edge case. base64-ish fields and odd file names happen all the time.

Tests

• added 2 regression tests for multipart text/file fields with extra =
• npm test --silent -w packages/insomnia -- src/main/importers/importers/curl.test.ts
• npm run type-check
• npm test
• npm run lint still reports existing warnings in untouched files, same as before

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