test(tss): added integration test for dynamic theme switching at runtime

[tmdeveloper007]

Summary of What Has Been Done

Added with 5 integration test cases verifying themes switch correctly at runtime.

Changes Made

• Created with 5 test cases:
  • switches active theme and returns correct values for dark and light themes
  • fires when is called
  • returns correct resolved styles for widget after theme switch
  • reloads engine when .tss file changes
  • osc-11 background detection triggers theme switch via

Impact it Made

• Ensures hot-reload of themes works without breaking active sessions
• Validates the theme variable resolution path
• Covers the runtime theme switching behavior that lacked test coverage

Closes Karanjot786#3291.

Note: please assign this PR to the tmdeveloper007 account.