fix: escape regex metacharacters in environment variable keys

[Fikri-20]

Summary

Fixes #1645

• Environment variable keys are now escaped with RegExp.escape() before being joined into the substitution regex
• Prevents FormatException when any key contains unclosed metacharacters like ( or [
• Also fixes silent wrong-substitution for keys with ., +, * (previously matched incorrectly due to unescaped regex semantics)

Root Cause

substituteVariables() built a single regex from all active env var keys without escaping:

// Before — crashes if any key has ( or [
final regex = RegExp("{{(${envVarMap.keys.join('|')})}}");

One malformed key corrupts the regex for every request in that environment, silently freezing the Send button with no error shown.

Fix

// After — keys are safely escaped
final escapedKeys = envVarMap.keys.map((key) => RegExp.escape(key)).join('|');
final regex = RegExp("{{($escapedKeys)}}");

Video for the app after resolve the bug

sol.mp4

now the current, and subsequent requests is sent appropriately!

Test plan

• Existing substituteVariables tests pass
• Manually verified: env var key api(key no longer freezes all requests
• Manually verified: env var key api.key now substitutes correctly (previously matched any char in place of .)
• Attach screen recording showing bug and fix (see linked issue [Bug]: Environment variable keys with special characters crash app with FormatException #1645)